I use one of these:
Waterproof phone mount

I use with Runkeeper to record my rides, but also so I can see my pre-planned route on screen. The only issue I had with it was that the mount goes up to 30mm & not 31.8mm - so I had to get a bar extension like this as well:
Carbon bar extension

I wish people would stop posting don't use a satnav, look at the scenery or you'll run into the back of a car type comments....we don't all live in well sign posted towns/cities. I live out in the countryside & ride on single track roads that i've never been on before with no traffic that generally have at best, poor signposting - more commonly none. Pre-planning my route in runkeeper ALLOWS me to look at the countryside, it doesn't prevent it.