Try placing some shadow areas down. Basically, once each picture has a dominant light source, in any place where the curvature of the object obscures light reaching it, darken the far end.
The fastest way to do this (probablly should only be used on low detail areas), is to take the basic color over an area, darken it fairly dramatically, and use the airbrush, set to a low percentage (5%-7% is what I use). Then, just brush it over the darker portions a few times.
There are much better methods for doing this, using floating selections and layers and other neat tricks. But I think the next step would be to lend images some more weight with shadows. It'll really help get rid of the sticker look.