Uninstall apps you don't need
Apps can take up a significant amount of room — especially games — so if you have icons collecting dust on your home screen then get rid of them (you can always install them again in the future). The easiest way to do this is to tap and hold an icon in the apps drawer then drag it up to the Uninstall button.
If you go into Apps from Settings you can see all the apps that are currently installed on your device. Tap the menu button (the three dots in the top right corner), then choose 'Sort by size' to see which ones are taking up the biggest chunks of megabytes (or gigabytes) on your device. Then delete as appropriate.
Delete pictures, music and video
This can be a time-consuming process, but work methodically through the media apps on your phone or tablet — do you have a large number of cached playlists on Spotify? Have you saved any Google Movies to local storage? Have you downloaded any media content files from a browser lately?
Remember that apps such as Google Photos can back up all your content to the cloud anyway, so you don't necessarily have to keep local copies. If there isn't a file manager already installed on your device, an app such as Astro File Manager or ES File Explorer can help you hunt down loose files.
Clear out app data
As well as uninstalling apps completely, you can also clean out their temporary data stores. In the cases of some apps (such as browsers or social media apps), the size of this cache can quickly grow, and you can free up a substantial chunk of space if you know where to look.
From Settings, dive into the Apps screen then tap on an individual app to see how much room it's taking up. You can clear both the cache and the local data storage using the buttons provided — though some of your app personalizations may need to be reconfigured as a result.
Use a third-party tool
There's no shortage of apps to help you clear some storage space on your Android device — and a lot of them are free and simple to use. They run through some of the procedures we've mentioned above, but you might find it easier to use them than doing everything manually.
CCleaner has a good reputation as a desktop app and it works well on mobile too, while Cache Cleaner Pro, Clean Master and The Cleaner are all also worth a look. Most of these apps go beyond the basics of freeing up storage space and are able to optimize other parts of your Android system too.