If you have a GSM phone with 3G capability, whenever possible use GSM. Using your cell phone in 3G or Dual Mode drains the battery much quicker than if you just use your phone in GSM mode. Usually buried somewhere in the specifications for your phone, they'll list the battery lifetimes separately for GSM and 3G. On most phones, the GSM lifetime will be 50% greater. If you need to preserve your battery life and you don't need the 3G capabilities, turn them off for a while.