One of our favourite products on the market currently is the Guitar Sidekick from Castiv. This allows you to attach your phone to the head of your guitar and easily see the iMoov app while you play.
However before the Sidekick was on the market we had a few other ways of attaching the phone to the guitar:
iPhone Armbands - Although they are designed for joggers, these can be attached to the body of the guitar as shown in the picture. There are many different styles of iPhone case that can be used in this way - we suggest you look for ones with the shorter straps.
Rubber Bands - these are way better than they first sound! If you don't want to spend money on extra gear, then you can simply get some striong elastic bands to attach the iPhone anywhere that you would like. See the pictures below...