If you have your phone enabled to search for 3G networks but you are not using the 3G features, then turning off the 3G on your phone will help in prolonging the battery life. You can do this by going to the network management menu and in network type, change to GSM only networks.
machan this happens when 3G is enabled, the phone tries to connect to the 3G network that is available in the region and sticks on to that network how ever low it may be. If a strong 2G network and a very weak 3G network is available, it will still connect to the 3G network. If only 2G network is available and 3G is still enabled on the phone, then it will continue searching till a 3G network becomes available. This searching and hanging onto the 3G network takes up a lot of the battery life. This happens even if you dont talk or use the phone.