iPhone 4 and newer iPhone battery replacement is fairly trivial:
1: Buy a battery and a pentalobe driver or bit from dealextreme or ebay for about $10
2: Uscrew the two case screws
3: Slide the back cover off
4: Unscrew the battery connector screw
5: Replace the battery and reassemble the back cover
I've done it about once a year on my iPhone 4, once the average recharge interval goes from about five days to about three days.