Just another thing to remember, do not force your heels down.
Often times when people are told to "keep heels down" they force them down and that spells all kinds of trouble. You need to let your weight naturally sink into the heels. A good exercise is to stand on a flight of stairs with your heels hanging over the edge. Stretch your heels all the way down, then lift yourself back up. Do it every day, several times every day until you can no longer do them; I promise you your heels will be perfect in no time. :)