There are several things you can do to extend the life of your battery between charges.
1. Set Email Filters. Yes that's right, set some email filters to block unnecessary messages received on your handheld. The fewer wireless transmissions your BlackBerry makes, the longer your battery will last.
2. Decrease the backlight timeout and/or brightness. This is especially true for the newer 7100, and 7290 handhelds. Go to Options --> Screen/Keyboard to set the backlight options.
3. Disable wireless sync on applications that you do not use, e.g. Memopad. Is it really necessary to transmit memopad data over the air? If not, then just disable Wireless Synchronization from the Memopad Options.
4. Turn Wireless Off when you are in areas of poor coverage. The BlackBerry will greatly increase it's output in poor coverage, and this can be very draining on your battery.
5. Set the Auto On/Off. Unless you're on call 24 hours a day, chances are you won't be using your BlackBerry late at night. So set the Auto On/Off to shut your BlackBerry off.
6. Set your profiles to have the BlackBerry vibrate and ring less often. Even just reducing the number of beeps can give you a little extra battery life.
As a last resort, if your BlackBerry battery simply doesn't last long enough, you can buy an extra battery