Design ideas on ways to lift things simply?

I am in the middle of a design project and my group is currently stuck on a mechanics problem. We are designing something with a shelving system. The shelves lower down when full but then need to be able to be raised back up later in order to easily unload its contents. Obviously with gravity the lowering part is easy but the raising is more difficult to do in a very easy way. Anyone have any ideas on how this could be done in a very simple way? (preferably without using electricity or motors)