2B, when you want to exit an application in the future, it would probably be in your best interest to press the blackberry button right next to the trackball ( I call it that because it has the logo on it) then scroll down to close to be sure you have exited the application your focused on. It will help with battery life too if you do this which means less recharging.
Most of the time when your phone locks up on you, a battery pull will solve it. Do yourself a favor though, download and install something like quickpull. There is a free version and will keep you from having to actually take the battery out on the chance you have a protective case on your phone. Saves aggravation more than anything.