For what purpose? Want to download more apps? Snappier response?
If you want more apps, 1) download a super plain flavor rom. One that has no bloatware and a bare minium like email app, no google maps, etc. Move as many apps as you can (while still mantaining the functions you want) to sd card. 2) if you already are on a rom you like first thing's first clear your cache from the applications menu. Back up your sd card and and wipe it clear and keep it bare eg only roms backups no/little wallpapers/pictures. Remove unnecessary apps and backups eg your old miui backup that you haven't used in three months. Delete texts as soon as they are unneccessary.
If you want a snappier response then keep your screens and widgets to a minium screens 3-5 max. Widgets maybe one per screen. Keep animation to none, keep your taskbar in home memory.