[abhem]

Im sure the obvious technique is to always use aluminum as opposed to steel whenever possible. Swiss cheesing is also a great choice. To save on weight, our team uses gusset plates to secure vertical and horizontal posts rather than using crossbars. Although crossbars are very handy to secure posts, gussets work equally well, if not better when made correctly. They also save a ton of weight and lots of physical space on the robot.
Using gussets to secure "swiss-cheesed" bars/c-channel/etc. is a powerful combo to save weight