The iPhone has the best cell phone camera I have ever used, but it isn't perfect. Here are some tips to keep your shots looking their best.
• Take a moment to see where your light is coming from. A bright light behind your subject will make your subject very dark, and you will not be able to see the details that you need. The iPhone doesn't have settings to keep the shutter open longer, so you need to use as much natural light as possible. Move your subject closer to a window or out of the shadows whenever possible. There is a built in flash for those shots where there just isn't enough natural light.
• Move yourself closer to your subject rather than using the zoom. Every photo I have taken with the zoom turns out grainy looking. I would rather need to crop some of the outside off with editing tools than have the entire photo turn out grainy.
• While you are setting up your shot on screen you can tap the screen to choose where the camera should focus. It only takes a second or two for the phone to re-focus, so I make it a habit to use this feature to be sure I am focussing on the correct subject. This is especially useful if you are using any of the composition techniques such as "rule of thirds" where your subject is not in the middle of the screen.
• Keep your camera lens clean. Just as with a point and shoot or DSLR camera, your pictures won't turn out well if there are smudges on your lens. Keep a small microfiber cloth with you to wipe your lens off whenever you pull your camera out of your pocket.
An iPhone could never completely take the place of a good digital camera, but can be a lifesaver for those moments you don't want to miss out on. Taking better pictures takes practice. Get out there and take more pictures with every type of camera you have!