My general approach to user interface design was simple: Grab a couple of assets and Photoshop and get on with it. A couple of weeks ago I participated in a little Hackathon. I had to realize I was quite slow designing a handful screens for quite a simple App. I occasionally glanced over to the team next to us. I was impressed. They used some fancy software and they were just pushing out fancy screen after screen after screen. So I've done a little research and I must say designing screens with Photoshop seems to be very 2001. Now there are different solutions:  https://www.sketchapp.com http://www.invisionapp.com http://www.axure.com/