A weekly battery pull will do this if you're experiencing slowdowns. Memory leak was solved almost completely with OS 4.6, and models newer than the 88xx series were nearly immune from the beginning.
Like NJBlackBerry said, the culprit was shoddily coded third-party apps. RIM had tightened it's requirements for new apps. There may be the odd theme out there that causes issues, but that's about it now.