When I have leftover paint, I have a couple of ways to handle this:
1) Sometimes, I get out a new gessoed board and use up all the paint, if possible
2) Cover your palette with a layer of plastic wrap (like Saran Wrap)
3) Lately, my best results have come from placing the mounds of leftover paint on a nonabsorbent surface (palette paper or freezer paper - the shiny side), covering with plastic wrap, rolling it up, folding the edges of the paper, taping it securely, and placing in a sealed zip loc bag. I've found this can last for at least a week, and even longer if I put it in the refrigerator (I recommend you keep this away from touching any food products, however!)