I'm very surprised no one has mentioned either of these.
http://www.chiefdelphi.com/media/papers/2250
http://www.chiefdelphi.com/media/papers/2175
The first is JVN's paper on Applying the Engineering Design Process to Competition Robotics.
The second is JVN's paper on using Weighted Objective Tables for Competitive Robot Design.
No matter what you decide to do I would read both of these papers. If you use these papers as your guide in the design process you will be happy with the resulting robot.
Sincerely, Bryan
(P.S. Seriously.. Read Them!)