Although I have never done it, I have seen it done with my teammates in college for the hurdles. Just take some athletic tape and roll it up so the sticky side is facing out. Make two of these. Then after you are in the blocks, have your coach lightly stick the tape to your heels. If you strike heel first before the last hurdle, the tape will come off, if not, then the tape stays on. It makes you stay on the balls of your feet and thus a lot harder to "break" and jump up.
Also, a big reason that most hurdlers will "break" and strike heel first is that they are not leading with the knee, but rather the whole leg. In order to counter-balance the weight of your leg coming up, you probably have a tendency to lean back thus striking in front of your COM.