you may be running low on memory with old stored messages and e-mails…
go to options > status > file free
Delete unused Languages:
1. Go to Options >> Advanced Options >> Applications
2. Scroll to and highlight the unused languages
3. Press the menu key and select Delete
4. Once all languages are deleted, remove battery for 30 seconds and re-boot the device.
and Remove call logs
Clear Cache and Cookies in Browser
actually what blackberry (BB) are you using? if your blackberry stores pictures, music and etc on the phone, get an 8bg or 16gb micro sd and move your files there.
apps that you don’t use can be removed too.
lastly do a battery pull if it still does not have enough memory. it will free up for a temporary time, but it might not help in a long run. if this steps dont help, maybe you will need to get a new BB phone
to clear your cache too you can use ALT + LGLG and you can clear all your logs that you dont need. or if you want u can set it not to log too.