First, why I are you standing on the board, out of curiosity?
Second, in a recent thread Rip revealed a neat little cue that he uses to get guys to arch their backs at the bottom of the squat: at the bottom, think about dropping the head of your dick down and back between your knees.
No joke. Try it and see if it works. Otherwise, you can try any of the number of tricks he uses to teach a hard arch and that are described in Starting Strength.
-Stacey