Magicpaddler, I like your idea of putting a rock in the bag. Will have to try that next year. 
This year I made a crude slingshot and it didn't work well-either- too much or too little so I went back to tying string around a rock and throwing it. ( and of course the string comes off at least once) .
In 2010 I went to Sawbill where I noticed the food pack up in the tree in front of Sawbill Outfitters hung using a pulley.  We tried that and it worked really well.  This year we rigged a 2 pulley system with one over the branch and the other at the food pack and it pulled up like it weighed 10 pounds.  That's my system from now on.  As I am only base camping now we only have to rig up once a trip.