Trees - kids seem to have problems w/branches...so I would have them look at their arms - like branches thicker at the "trunk" and smallest at the ends ( like fingers).The size analogy seemed to work pretty good. Also - the lower branches are to the ground the more parallel they are...at top of tree they point to the sky. A little "Eric Jensen" approach - get them out of their seat and model this might do the trick! I used this at the elementary level - but middle schoolers might like it.
RE: bases for sculptures...any parent's who are contractors....line them up for decking scraps....or 2 x 4 scraps...I have our tech teacher cut them up for the wire/stocking sculptures. Same issues...kids tend to toss them out. Another thing...getting kids to VALUE their work....