Paper prototypes are my favorites. Making them is as quick as it takes to write on a piece of paper and cut out some cardboard elements. Each time is like creating a little board game! Here's an example:
A simple sketch on a piece of paper might be a great start for a prototype; in this case, the emphasis is on the touch inputs for an RTS game
If you're wondering what a paper prototype might look like, have a look at the following screenshot:
A paper prototype for an alternative combat system for Battlehand: Heroes. We're ...
