I have lots of friends inheriting 2nd hand iPhones (3G / 3GS models).
Here are some tips for extending the battery life of the phone throughout the day…
1. Got to Settings > Wifi > Turn off auto discover wifi networks.
2. When your not at home or not using it turn off Wifi off. You really only really need it on when you downloading apps or doing some heavy web browsing with lots of images. Facebook, Email, Twitter really won’t use much of your data allowance so it’s almost not worth having on.
3. Settings > Brightness > Try and keep it as low as your eyes can tolerate.
4. Settings > General > Network > Enable 3G… I used to turn off 3G quite a bit unless I was doing heavy web browsing as mentioned above… if you just retrieving text (ie .Facebook, Twitter & Email then the 2G network is often sufficient in built up city areas…. this one saves LOTS of battery.
5. Settings > Mail > Fetch new data… If you are not using ‘Push’ email, changing fetch interval from 15 minutes to 1 hour will help also. Not using ‘Push’ also helps, but the convenience of the push services probably outweighs the battery drain.
6. If you do like 3G on all the time, just turn it off when the battery gets below 20%… will give you 1-2 hours more time before the phone dies.
Just remember it may seem like the iPhone has a crappier battery than your old Nokia or Motorola, but did you use that old phone as much as this one?… me thinks probably not :)