The correct way to charge any Blackberry is to let the battery get as low as you can (10-15%), then turn it off and plug it in and let it fully charge (this does take about 2 hours). I was charging my phone with it on! oops.. If you are charging the phone when ever and plugging it in with the phone on it will make the battery think its full life is up , and you will start having the phone run out of juice with hardly any use.
I have always charged my battery when its pretty low, but there have been time when I knew I had to have it and charged it anyway. Don't do this because you will end up needing a new battery.
hope this helps someone out there :)