You need at least 15MB of free memory for normal operation.
You should delete unused languages, the sample video, any unused games, transfer pictures you no longer need on your phone to your PC (or delete them), and reduce the memory allocated for picture storage from the options menu while you're in the picture section.
You should also periodically empty your log by holding down the alt key and typing LGLG, and then using the options to clear it.
Also, read these: Why is my 8100 Pearl losing its call logs or message logs? How do I delete the preloaded media from the device? How do I delete applications from my BlackBerry?