iPhone 3G / 3GS / 4
Beware of fake batteries easily available everywhere on the web. Your iPhone will be reporting wrong battery status, sudden shut downs and when restoring your iPhone you get an error 29 message.
To take care of Li-ion Polymer batteries, ensure that there is charge. DO NOT let your battery go to zero.
Once a month, allow the battery to drain below 20%, charge it up fully to 100% and leave it for 2 hours. This trickle charge will ensure it reaches full charge status.