I remember building one of these in high school. I agree with what Patrick said, he could have been describing the car I built. One thing I would add is a multi ratio axle. You can wrap most of the string around your normal axle, then wrap the end thats going to unwind first around a larger section of the axle. The mouse trap arm has better leverage on the axle the closer it gets to being perpendicular to the string. You can take advantage of this increased leverage by using a longer lever extension and getting more distance, but if it doesnt have enough torque to get moving when the arm is almost parallel to the string, you wont win.
Another thing is tracking. This sounds stupid, but if its not going straight you probably wont do too well. And since you probably wont get all the angles/ measurements perfect when you build it, its a good idea to design in some adjustability. Sort of like a steering trim.