I'm a night shift taxi driver and and usually run the battery on my iphone down every shift from tethering it to my laptop. I keep a 12v charger with me, but it is better for the battery to wait til it hits 10% then plug it in or just leave it plugged in all night?
I think the consensus with battery life is to generally just let it completely die before recharging. I usually plug my iPhone 3 in at the 20% alert, but I also usually get around 3-4 days out of a charge. I keep GPS off, 3G disabled, roaming disabled, etc. I'm in the minority though, I'm not tethered to my phone 24/7 (don't do social media, rarely get phone calls, etc.). Most of what I do is texting and that's only because of all my coworkers. For what it's worth...
Just a heads up to help out in prolonging your battery on ur iPhone... Double-tap the home button and you'll see all of the apps that you have used. Tap and hold until you see the apps start to wiggle and hit the minus button to close them.
I'm a bit of a computer tekkie and despite popular belief... Even though you are not using them they are still running in the background. Basically anything that you keep open and don't use are occupying memory thus utilizes battery life. So if you don't know of this trick already... It should also help keep ur phone alive when you're running low on battery life. Tchuss!