I usually don't arch that much doing dumbells, but I do make sure that my upper back is pinched together. When you pull your shoulder blades together it makes you a little more stable.
When benching with a bar I get an upper back and a lower back arch. I recommend buying the metal militia benching video that is usually on sale at ebay for 25 bucks or so.
I am up on my toes when I bench and like wnt2 said, I try to drive my heels into the ground but usually never get there. That is a good way to get leg drive when you bench. If you are on your toes too much and don't have balance, like bst said, move your feet out and a little more forward to stabilize yourself.