I used to use AutoDisconnect, but I like tinkering so I'm now using 3G/2G/Dual Mode Selection Applet instead.

Whenever I don't really need the speed, I use 2G for IM and IRC and Email and such. 3G is a huge battery hog, while on 2G I can idle on IM for the whole workday and lose maybe 10% battery capacity or less.

Sometimes in a poor network area (forests, islands, etc) I use 2G for better signal, as phones tend to use more power in poor signal areas.

I also recommend installing Wifi Switcher and turning off Wifi module whenever not using it.