It's a perennial problem, running out of space on your Mac's main disk. And if you have a relatively modern Mac with SSD storage rather than a hard disk, since the capacity of SSDs (or Flash storage) tends to be smaller.
Even a MacBook Pro with a 512GB SSD, like the one on which I'm writing this, will run of storage eventually. Also see: Best SSD for Mac.
Updated 5 September 2016 with pricing updates.
How to free up disk space on your Mac: Remove photos to free up hard drive space
The first thing you should do when you decide to free up space is pick the low-hanging fruit, ie those files which occupy tens of gigabytes of disk space on their own. One good place to start, particularly if you've just migrated to the new Photos app, is with the files created by Aperture and iPhoto. Both those apps stored their libraries in a single file, located in Pictures in your user directory.
When you migrate to Photos from Aperture or iPhoto, the old library files remain in your Pictures folder, meaning you now have a Photos library and an Aperture or iPhoto library. If you never plan to use Aperture or iPhoto again, you can get rid of their libraries, but we'd recommend archiving them on an external hard drive just in case (Note, there can be some confusion about your photo library in Photos and iPhoto - the apps are accessing the same images, so your whole library isn't duplicated, but there are associated files that you no longer need).
Once you've migrated all your photos to the Photos app, go to System Preferences, then iCloud and make sure Photos is ticked. Click on the options button and make sure iCloud Photo Library is checked. You may need to pay for additional storage in iCloud if you have a large photo library, but it's not too expensive: 200GB costs £2.49/ month.
How to free up disk space on your Mac: Crop iTunes to recover storage space on your Mac
Your iTunes library is another candidate for re-claiming disk space. You have a number of options here. You can copy the whole thing from your Music directory to an external hard drive and point iTunes to it from its Preferences. That's great if your Mac is a desktop model, but not ideal if it's a notebook. You could use a NAS box instead of an external hard drive in that instance, so that you can access your music whenever you're connected to your local network.
A third option is to pay £21.99/year to subscribe to iTunes Match. Once you've set it up, iTunes Match allows you to access all the music in your iTunes Library on Apple's servers, meaning you don't have to have it stored locally at all. You'll need to be connected to the internet in order to play music, but other than that, it's just like using iTunes with locally stored music. And, as a bonus, if you decide at a later date that you want to download your music from iTunes Match, you get 256-bit AAC files which are probably of better quality than the ones you had stored on your Mac.
How to free up disk space on your Mac: Remove files from your Downloads folder
Your Downloads folder also probably houses large files you know longer need. If you download large PDFs, images, or disk image files and don't prune Downloads regularly, it's probably full of stuff you don't need. Start with disk images. If you've installed their contents, you won't need them anymore. If they contain apps, you should always install the latest version anyway.
How to free up disk space on your Mac: Find out what apps are hogging space on your Mac
Once you've dealt with the obvious culprits, it's time to go deeper and find the other disk hogs. There are several apps that will show you which files are taking up big chunks of disk space, or allow you to order files in the Finder based on their size.
GrandPerspective (free) and DaisyDisk (£7.99, buy it here) give good visual indications while OmniDiskSweeper (free) uses the standard hierarchical file window to show the sizes of every file and folder. Other apps such as CleanMyMac 2 (£34.95) show disk usage as part of their cleanup features.
How to free up disk space on your Mac: How to see what files are taking up the most space on a Mac without buying an app
You can also do it easily without an app. Open a new Finder window and navigate to your Home folder, or press Shift-Cmd-H from the Finder. Press Cmd-F to open a new Find window and in the left hand dropdown menu at the top of the window, choose Other. From the window that opens next, scroll down until you see File Size, and check the box next to it. You have now selected that as the attribute for a search. Select 'greater than' in the next dropdown menu, and change the file size units to MB. Now type in a file size, say 100, to display all the files in your Home folder that are bigger than 100MB. You can now choose which files to delete or archive on an external disk, and free up disk space quickly.
How to free up disk space on your Mac: Empty the trash to recover space on your Mac
Empty the Trash frequently. If there are any stubborn files that won’t disappear, the easiest solution is to use Trash It! (free), a useful one-trick pony.
How to free up disk space on your Mac: Delete attachments from Mail to save space on your Mac
How about your Mail attachments folder? Think about how many emails you receive with large attachments. What happens to these? Within the main Mail folder (~/Library/Mail), attachments for incoming mail are stored in the inbox folder. To make sure those from deleted emails are removed, go to “Remove unedited downloads” in Mail’s preferences and select “After Message is Deleted”. The current version of Mail also stores attachments in ~/Library/Containers/com.apple.mail/Data/Mail Downloads. As long as you don’t need them these can also be deleted, often saving you many hundreds of MB.
How to free up disk space on your Mac: Remove duplicate files from your Mac
Identifying and dumping duplicate files is another good way of freeing up disk space. Gemini costs £15.95 on the Mac App Store (in the US you can buy it on the Mac Store for $19.99) and allows you to scan your Mac for duplicate files so you can dump one copy.
How much of your precious hard drive space is being taken up by duplicate files? While hard drives may be getting bigger and cheaper, Apple’s direction is that of smaller, faster SSD drives. The problem is that these have smaller capacities so a higher level of filing discipline is essential.
There are a number of reasons why you end up with so many dupe files. When you add songs to iTunes, if you have ‘Copy files to iTunes Media folder when adding to library’ checked in the preferences, you keep the original. Instant duplication. A couple of thousand high quality songs and that’s anything up to 10GB hard drive space wasted. And that doesn’t include dupes within iTunes.
If you use Apple Mail, remember that all attachments reside in Mail’s own download folder. This can be another source of duplication.
More generic apps use various strategies and criteria to find duplicates and give you control over which ones to get rid of. Have a look at Tidy Up ($29.99) or Singlemizer ($19.99, buy it from the Mac App Store here).
How to free up disk space on your Mac: Quit apps that are running in the background to free up space
Quitting apps that have been open for several days or more, or even restarting your Mac completely on a regular basis will also help free up disk space. Applications create temporary files to store data and the longer they run without quitting, the bigger those files become. When you quit the app, the cache files are deleted and the disk space returned.
How to free up disk space on your Mac: Get rid of unwanted Mac apps to save space, delete associated support files
It’s amazing how many apps you can acquire over a short period of time let alone over the many years most of us have been using a Mac. Most Mac OS X apps are bundles. The application is usually a special folder that looks like a single, double-clickable file and contains almost all the files needed to run the app. Trashing the app is easy; getting rid of all the support files isn’t. There are preferences (plist) and application support files and these can exist in a number of places on your Mac. These files along with the app itself may be wasting many gigabytes of your hard drive space.
Some major apps include an uninstaller. For example, you’ll find one of these in the Additional Tools folder of Microsoft Office. Sometimes an app’s installer doubles as an uninstaller. But the lack of a dedicated uninstaller in Mac OS X is a serious omission.
Fortunately there are a number of third party options. AppCleaner (free, download from the Mac App Store here), AppDelete ($7.99, there's a Lite version on the Mac App Store here for £2.99) and AppZapper ($12.95) all do the same job – but the fact that they each find different files to remove shows how complicated a process uninstalling can be!
How to free up disk space on your Mac: Remove extra languages you don't need
Mac OS X supports a range of languages, being localised for over 25 languages all of which are included automatically during installation. Go to System Preferences > Language & Region, here languages can be put into preferred order making it easy to switch between them. Many major applications support multiple languages too, using the order from Language & Text to select one if the app doesn’t support your main language. The problem is that if you only want to use one or two languages, Mac OS X and many of your apps are bloated with all the others.
If you want to delete extra language files that you know you won't be needing, go to the Resources folder and look for folders ending in .lproj. Each of those folders will include a language file. You should be able to trash these folders withouth any problems.
How to free up disk space on your Mac: Remove unwanted code to save disk space
Getting rid of unwanted code is another disk-saving exercise. Universal Binaries host both PowerPC and Intel architectures yet since Lion, the former is not supported.
Monolingual (donation requested) lets you remove specific architectures from Mac OS X along with specific languages. While it can claw back huge swathes of hard drive space, unless you’re careful with the preferences it can also render your Mac unbootable so use with extreme care.
Xslimmer ($14.95) works on an app level, keeping only the languages you select. Again, care is required as some apps view the removal of code as being an attempt at circumventing copy protection. To that end, Xslimmer keeps an updated blacklist of apps to be left alone. It shoudl be noted that it has since been discontinued.
How to free up disk space on your Mac: Use Cloud storage space to suppliment the capacity of your Mac's hard drive
Cloud storage services are great for making files available remotely, but they can also take up space on your Mac. Both Dropbox and OneDrive for example, sync everything you store in them with your Mac by default – assuming you've installed the Dropbox/ OneDrive app. If you only have the default 2GB storage available for free from Dropbox, that's not too much of a problem. But Microsoft gives Office 365 subscribers 1TB of space free, so if you use that to store lots of files, you could find yourself running out of disk space very quickly. The same is true if you pay for more space on Dropbox.
In both cases, however, you can choose to sync only files and folders you specify. In Dropbox, click on the menu bar item, then click the cog and then Preferences. Click the Account tab, then click Change Settings. Now untick the files and folders you don't want to sync with your Mac. In OneDrive, click the menu bar item, then Preferences. Click Choose Folders, then Choose Folders, then Choose Folders to sync. Untick folders, or click the expand arrow to access individual files and untick those.
A alternative would be to sign up for more iCloud storage, as we mentioned earlier, 200GB of iCloud storage costs £2.49 a month, £6.99 a month for 1TB, or £13.99 for 2TB, there's also a 50GB option for 79p a month... And it's easy to access from any of your Apple devices.
I need the space, but I don't want to delete anything!
But I really need to keep everything!
If you really are the proverbial data squirrel, here are a few simple suggestions:
• Archive any files you are unlikely to need regularly. Control-click on a folder and select the compress option. The space saved will vary according to the type of file being archived: JPEGs and DMGs, for instance, are unlikely to compress very much. Once created, archives can either remain on your Mac or be saved to an external drive.
• Use an external drive for data files. This would be especially useful for design, audio or video files.
• Migrate your iTunes Music folder to an external drive and then relink to this within iTunes. This is quite possibly the single largest folder on your Mac: mine is a disk-busting 25GB.
Finally, if you do take the decision to delete files or folders, always back them up first. You never know…