Have you installed a boat load of tweaks (*in cydia) and now your storage is down to 0? Well here's how to fix it:
1.) If you were lucky enough to have Icleaner installed before you filled your storage, go to the app and press: Move languages. This will give you another 100+ MB to your storage.
2.) If you weren't so lucky, you can restore your device via iTunes OR with a program for windows called Semi-Restore, which will allow you to get a fresh start without having to rejailbreak.