One of the keys to getting good pictures is to have your camera be very steady when you are taking the picture. One of the easiest ways to mess up the steadiness of your camera, is by pushing the shutter button too hard and thus jiggling the camera.

One thing I noticed on the iPhone is that it does not take the picture when you press the button if you leave your finger on the button. Instead, it will take the picture when you take your finger off the button. I find this to be a much gentler procedure (removing my finger versus pushing then letting off) and it results in less movement of the phone and the result is better pictures. If you consciously think about letting off the button to take your picture, I think you'll find that you get better pictures with your iPhone.