Personally, I keep of my password and other propietary data in the Password Safe that is part of the BB OS. I alos have my device set to lock in 5 minutes. Doesn't seem to be a particular drain on my devce, and after a while, you get ysed to popping in the password.
There are also some third party security apps that have gotten some good reviews. Check out the Aftermarket Siftware section, there may be some info there that will assist you: Aftermarket Software - BlackBerryForums.com : Your Number One BlackBerry Community