Wet Pallet

The first thing to do is go to your art store and get some Acrylic paper that is made for wet pallets.  If you don't have the money or just can't get to a store that supplies it then wax paper should work.  Get a tupper ware container, one of the Glad disposable ones work well, cheap and you can buy them at almost any supermarket. 

Now get a sponge, again you can get one that is for wet pallets at the art store.  But you can use a regular sponge no problem.  Now just get the sponge wet, not dripping water all over, but wet enough to keep the paper moist.  Wet the paper and then place it on the sponge and you are all set.  If you have the lid for the container you can keep it in the fridge for many days while you are painting.  This works for not wasting paint, not having to match the mixed color and your paint doesn't dry up before you get a chance to paint :)