Turn off the GPS time setting, turn off blue tooth, turn on auto brightness. Also don’t leave apps running in the background if they’re actually doing something (e.g. don’t leave a GPS tracking app running in the background, but facebook is fine).
To kill a background app, double click the home button, you’ll then get a list of apps appear from the bottom. Then hold your finger on the apps which will make a little red cross appear on them, then hit the cross.
I get about 48 hours with my 1.5 year old Iphone 4. But as pointed out, if you’re on the web all day you’ll get less. When I just use it as a phone and turn data off (when on holiday) I can get a week.
I’d keep the 4S as in my experience android phones are just as bad if you use their features as much – that’s the problem with having easy to use features – you use them!