How do I prevent popping up in my 40 yard dash start?

If you have access to a sled (one that is used for resisted running), they are great for working on forward lean during your start. Do not use more than 20-30% of your body weight as resistance and do not perform more than a 10 yard sprint, as this can lead to altered stride length and possible hamstring issues. Start low and powerful and lean forward into the belt that attaches you to the sled and work on leg drive and staying down. The resistance of the sled will allow you more lean thus helping create better launch technique.

You can lose proper posture in your starting stance when you transition to a full sprint if you fail to keep your back flat and create a forward lean from your ankle. When you round your back, you lose power and with lost power - your chest comes up. If this is the case, a great exercise to perform in your warm-up is the lean, fall, jog. With this exercise, you need to keep your back straight and bend at the ankles until you feel like you are going to fall on your face. At this point, fight to maintain posture by keeping the abs really tight and break into a smooth jog. You do not have to jog too far, the real work is done on the starting line.