For last year's game, our team prototyped parts of the robot (most notably the robot's arm) with some cheap wood we had lying around. Once we were satisifed with how everything was working, we went to the aluminium. This worked pretty well because it gave the people working on the drive train and electrical systems time to build and debug their systems in parallel with the arm people.
On a more comical side: One team member had a few too many K'Nex on his hands, so he buit a little mock up of the planned robot, and the goal. *That* was a riot!
