Just pop off the back cover and yank the battery out.
Seriously, that's all there is to it. The old Palm Treo phones had a reset hole you had to poke with the tip of your stylus. BlackBerry does it by taking out the battery while the device is powered on.
If you do this on a regular basis you might consider an app to restart your BB automatically. Quick Pull Pro
does this for you, and you can schedule it to happen overnight, while you're asleep.