Retaining the quality of your iPhone battery will save you a fair amount of money in the long run, so I would like to take a moment and offer some tips and advice on iPhone battery care and how you can optimize the life and charge of your battery to enable you to keep that cash in your pocket for as long as possible.
So the first thing to mention for iPhone battery care would be a proper charge cycle, a correctly maintained iPhone battery can retain up to 80% of it´s full capacity for 400 full charge and discharge cycles, therefore it is important to not overcharge, or undercharge the battery whenever possible. Allowing your iPhone's battery to run completely flat once in a while is great, but at other times allow it to run as low as possible before charging it again, and when you do charge try to achieve the maximum charge replacing full life back into the battery.
Achieve the maximum ability from your battery - this will not only help you monitor the life of the battery, but also take us back to tip one and ensure a good charge cycle is in operation. For example an iPhone 3G genuine battery will offer you 12 hours of talk time, 10 hours of video viewing, 9 hours of Wi-Fi use or 30 hours of audio playback from a full charge, try to be aware of how your iPhone is being used and how the battery is standing up to the task - also bearing I mind that standby time is somewhere around the 300 hour mark.
Allow the battery to breathe - if you are charging your battery in some carrying cases they tend to generate excessive heat which can affect the capacity of your battery, if you are aware that your iPhone is becoming warm during it´s charge remove it from it´s case prior to commencing the charge.
And one last pointer, is try to generate regular use from your phone, the lithium-based batteries perform better when the electrons are kept active inside, and make sure that whatever happens with your charging the iPhone battery receives your full care by giving it a minimum of one full charge (100% of the battery life) per month.