Assess the situation
Like PCs, Android devices may behave unpredictably as they run short of storage space or memory. Symptoms that this may be happening on your device include apps that randomly crash or won't load at all, and an inability to save new pictures or videos. To see whether you're running low on space or memory, open the storage and system monitors built into Android. (Note: The menus I refer to may differ cosmetically from those on your device, depending on the version of Android you're running, but accessing them should be similar.)
First, bring down the notification shade or open your app drawer, and tap the Settings icon--it usually looks like a gear. Scroll down the long list of settings until you'll see a menu item labeled 'Storage'. Tap the Storage icon, and you should see an informative list that details the amounts of storage space currently occupied by the device's applications, pictures and video, audio files and ringtones, downloads, and miscellaneous files--along with entries indicating the device's total storage space and its remaining available space.
In the Storage menu, you can monitor how much storage space is left on your device, but you can't do much else except format a storage volume--which you won't want to do unless you're wiping the device clean. To make changes to the device without wiping anything out, you'll need to use Application Manager, Android's built-in app manager. To access it, go to Settings, scroll down the list of options to Application Manager, and tap it (on some devices, you may have to tap Applications and then Manage or Manage Applications). With Application Manager open, you can swipe to reveal three columns of apps: Downloaded, Running, and All.
The list of Downloaded apps will show all of the apps you've downloaded from the Google Play store, as well as many of the stand-alone apps that your carrier or device manufacturer installed. The Running apps and All apps lists are self-explanatory. At the bottom of the Download and All columns, you'll see the amount of storage space that the app is using; and at the bottom of the Running column, the cumulative amount of memory that all of the apps are using appears.
Tapping an app in the Downloaded or All column will reveal a menu with various options, including choices to force-stop an app (basically, killing it to free up memory), to uninstall it, or to clear cache and app data. If you want to remove an app, simply tap the Uninstall button. If you're trying to rehabilitate an app that's no longer running properly, however, the 'Clear cache' and 'Clear data' buttons may resolve the issues. Tapping the 'Clear cache' button will wipe out any cached data or files associated with the app, and force recaching of fresh copies. Tapping the 'Clear data' button will delete all personal data associated with an app, including login data and high scores for various games. The app should return to its freshly downloaded condition. If an app malfunctions, first tap Clear cache. If that doesn't help, tap Clear data. If that, too, fails to solve the problem, try uninstalling the app (by tapping Uninstall), restarting your device, and reinstalling the app.
Removing and relocating apps and data
As I've mentioned, uninstalling an app is as easy as opening Android's Application Manager, tapping the app in the All apps list, and then tapping Uninstall.
Unfortunately, some apps that the manufacturer preinstalled on your device may be embedded in such a way that you can't uninstall them unless you have root access to the phone or custom firmware.
In lieu of uninstalling apps to free up internal storage space on your device, you can move apps to a MicroSD card. Newer devices running Android Ice Cream Sandwich (ICS) or Jelly Bean (JB) may not let you install apps on an SD Card, but older versions of Android have the SD Card installation feature built-in.
Before you move any apps, though, be aware that any app that has an associated home-screen widget or that requires access to certain Android system files won't function properly when installed on an SD Card. If you're unsure about which apps you can safely move, you might want to download an app from the Google Play store called App 2 SD. This simple tool scans the apps installed on your device and lists the ones that you can transfer to SD Card without hazard.
To move an app, you can tap it in App 2 SD, which will open the app's properties in the Application Manager. Once there, tap Force stop and then tap the Move to SD button available in the Storage section. After a few moments, the app will reside on your SD Card.
Of course, unless you have a ton of them installed, apps probably aren't the biggest storage space hogs on your device. Photo and video files tend to take up much more space than apps, so it's worth moving them to an SD Card as well. To ensure that your device stores new photos and videos on your SD Card automatically, open your camera app and navigate to its settings menu (this will vary from device to device).
In the settings menu, navigate to the Storage section, and change the option from 'Phone' or 'Internal Storage' to Memory Card. If you'd also like to move your existing photos and videos, you can easily do so by connecting your device to a PC via a USB cable; the PC should recognize your phone or tablet as a removable storage device, at which point you can move the picture folder from the device's internal storage to the SD Card by dragging it over.
If you would rather complete the process on the device itself without using a PC, open the file manager included on your device (if your device doesn't have one, you can find plenty of them in the Google Play store) and browse to your picture/video folder. Tap and hold it, and in the resulting menu, select Move. Then browse to your SD Card and tap Move Here.
If your Android device won't boot properly or is otherwise unusable, clearing the device's cache partition or restoring it to factory defaults may be your only option. Android-based smartphones and tablets usually come with a built-in recovery tool that you can use to perform some maintenance operations or to restore the device's software to like-new condition. The procedure for entering recovery mode varies from device to device, but it usually entails powering the device down, and holding some combination of buttons while powering the device back on. On a Samsung Galaxy Note II, for example, you launch recovery mode by powering down the phone and then holding down the home and volume up buttons while simultaneously pressing the power button.
Once your device enters recovery mode, you'll see a basic menu containing a few options for applying updates or wiping various partitions. In most instances, you'll navigate the menu with your device's volume up and volume down buttons, and you'll press the power button to select an item. Before wiping all data and performing a factory reset, it's a good idea to wipe the cache partition alone, since this less extreme step occasionally resolves stability issues. Wiping the cache partition removes app components, temporary files, and other random bits of data stored in the cache, but the cache will automatically rebuild with fresh data when you reboot your phone and begin using your apps.
If after trying every other fix described here, you still have issues with your Android device, wiping the cache and data partitions and returning it to its factory presets will restore the device's software to like-new condition. But consider this your last resort: The process of wiping data and restoring factory presets is akin to the process of reformatting your PC's hard drive and installing a clean copy of Windows. You'll lose every bit of personal data stored on the device's internal memory and will have to reinstall all of your apps.
If that outcome is acceptable to you, enter recovery mode on your device and choose the options to wipe cache and wipe data. When you reboot your device, it will behave as though you were powering it up for the first time.