I judge the products based on their mediums. My cream, gel, and emollient based products stay seperate as to not contaminate, so I don't share mascaras, lipglosses, or foundations especially. For jobs I have mascara wands, gloss wands, and little jars for foundation mixing. Sometimes I carry around index cards to put the excess on and then just toss them at the end.
As far a powder products go, I share between my stash and jobs such as blushes and e/s, but make sure to wipe down the shadows in between uses and ALWAYS use CLEAN brushes!