Make sure that you have the most current version of RIM's software. If you do and this is something that happens daily, backup your files via blackberry desktop manager, perform a master reset on the device by wiping the entire device clean then restore your files to your phone and see how it works. You will need to reprogram your phone after you complete the master reset.
If the problem continues and you have insurance/extended warranty on that device then call customer service so you can get a replacement phone. Otherwise, look to upgrading to the blackberry tour, which is a much nicer phone with a 3.2 mp camera! =D
Are you completely shutting down apps when you're done with them and clearing your memory periodically? In addition to the things the other poster said, You might want to run the memory cleaner frequently, and assure all apps are shut down or exited completely. I try to follow those procedures followed by the occasional battery pull to keep my running smooth without lock-ups or glitches.
I also recommend cleaning your event log daily. Hold the alt button and type L,G,L,G. hit menu and clear log.
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