Talk about an inconvenient truth: right around the 12-month mark, your iPhone battery will start to wear out. You'll notice that it doesn't last quite so long between charges, a problem that just gets worse as more time passes. By 18 months or two years, you'll be lucky to get half a day of run time.
Apple famously (make that infamously) engineers iPhones without a battery door, so you can't just swap in a new power pack. However, you can indeed replace your iPhone's battery, and it's cheaper and easier than you might think.
In fact, I just put a new battery in my 15-month-old iPhone 4S, which was definitely showing signs of charge wear. Total cost: $8. Total time: 20 minutes.
Let me pause right here to note that opening your iPhone will void whatever's left of your warranty (which, unless you extended it, expired after 12 months anyway). I should also note that this procedure should work more or less the same with an iPhone 4 or iPhone 3GS. I'm not sure about the iPhone 5, but most folks are at least eight months away from having to worry about that.
I should also note that any time you go monkeying inside a device as compact and sophisticated as an iPhone, you risk damaging it. My upgrade went smoothly; your mileage may vary. Proceed with caution.
To find a replacement battery, I simply searched eBay for "iPhone 4S replacement battery." A number of top-rated sellers were offering that product, along with a mini-tool kit, for $8-$10. I chose one priced at $7.99 shipped; it arrived on my doorstep five days later.
Next I went to YouTube, searched for "replace iPhone 4S battery," and watched a couple how-to videos before even picking up a screwdriver. (One especially good pick: This how-to from DirectFix.com, which is also embedded below.) I wanted to learn the procedure as well as I could before getting started.
Finally, armed with a desktop magnifier (an essential tool for someone with aging eyes), I removed the two pentalobe screws from the bottom of my iPhone, slid off the backplate, then removed the two screws holding down the battery connector. It was fortunate I'd watched the how-to videos, otherwise I'd never have known what to do with the tiny little bracket that popped loose from I-wasn't-sure-where.
From there it was a simple matter to pry out the old battery, drop in the new one, and put everything back together. Much to my relief, my 4S immediately returned to life, and battery life is, as expected, much improved.
You could always hire a service to install a new battery for you, but that usually involves shipping your iPhone, waiting at least a few days to get it back, and paying around $40. For as little as 8 bucks, you can do the job yourself without letting your phone out of your sight.
If you've already done so, hit the comments and let your fellow readers know how it went.