Like most teams- brackets that "sandwhich" the frame. Some holes and these pins for $3 each.
http://www.harborfreight.com/square-...pin-96281.html
One big change I plan for this year is to have handles to lift our robot. The last 3 years the students lift by the bumpers. After a day or two the brackets are bent, holes don't line up, and it becomes a pain to remove and replace the bumpers. Bumpers strong enough for competition are not strong enough to be a lifting point.
Troy