NEWS FLASH! The best selling camera in the world is...
The iPhone. You (or someone you know) is probably carrying one, using one, posting to instagram this very minute. What a lot of people don't realize is the the iPhone can make great prints. I've made 11x14 prints off my iPhone that look great. Not everyone has had the same experience. So let me tell you my secrets for great prints from the iPhone.

Start with a good file.
The iPhone 4s and 5 both have 8MP sensors. While that's not huge by today's standards, it's plenty big enough to pull some nice prints from at full resolution. The Apple Camera App always shoots in full resolution, but if you use another App check your settings. If you're emailing it to someone who's going to print it, don't shrink it. Send it full resolution.
Keep it clean.
Because of the way we carry and handle our iPhones, the lenses are usually FILTHY! Carry a lens cloth and clean the lens before every shot or shooting session. No cloth? Well pick one up next time you're in, but a soft, clean t-shirt or other clothing item with work in a pinch. Avoid carrying your iPhone in the same pocket with change, keys or anything else that can scratch or damage the lens.

Crop now, not later.
The iPhone's have great zoom and crop features, but if you want to makes prints (as opposed to just posting online) try to avoid using them. When you zoom in you're cropping in on the sensor, the further you crop in, the lower the resolution. Cropping the photo has the same effect. The Moral is zoom (and crop) with your feet. Get in close enough to compose the picture the way you want it on the screen before you take it. This is where you'll retain enough resolution to make those bigger prints possible.
Spiff it up a bit.
Not every picture comes out just the way you wanted it too, so most iPhonographers will have a few apps at the ready to make the pics perfect. There are a ton of photo editing apps out there. Prices can run from free to around $15. Some of the apps I find myself using the most are: Snapseed, Photowizard and PaintFX for general and some special effects or masking. Dynamic Light has some nice HDR (high dynamic range) effects, and Tiffen PhotoFX has some great diffusion effects. For a more in depth discussion of photo apps check out Sandra Nykerk's next iPhoneography class.
...and of course...
Use a good lab. Download the F-11 Photo Print & Share appand upload those shots directly from your iPhone to the best photo printers in the west. (maybe anywhere)