On my old G1 I had to use the task killer to make sure that programs weren't hogging up all my batter life, but on the mt3gs I have not had to do this. The main three things that kill the battery on the slide are; the screen, the WiFi, and the biggest hog: the GPS!
What I did was put widgets on my desktop that allowed me to turn on and off the WiFi, the Bluetooth, and the GPS as well as see their status (on or off).
To do this follow these steps:
1. On the home screen press the menu button with the three horizontal lines.
2. Press "add to home" plus sign icon. (You may get a message saying "You home screen is full", make sure that you have room for three icons, one for Wifi, Bluetooth, and GPS)
3. Click Widget.
4. Scroll down to "settings" and click it.
5. You will see widgets for; Airplane Mode, Bluetooth, GPS, Mobile Network, Wi-Fi. Select the GPS and it will be added to your desktop.
6. Repeat steps 1-5 adding a widget for Bluetooth and Wi-Fi as well.
This makes it really easy to see if your GPS and WiFi are on and allows you to turn them off when not in use.
I recommend that you only have the WiFi on when you want to surf the net and you are in range of a WiFi access point that you can connect to. Having WiFi on all the time WILL drain the battery because it is always searching for access points.
Same thing with the GPS. Keep it off unless you are planning to use the Navigation or My Location in maps.
After installing these widgets I have had no problem with battery life. I just charge my mt3gs every night when I am sleeping.
Hope this was useful!