The answer to this question depends on where you are when the ball is hit at your feet. If you are on the return side than you need to change your return of serve (put a little more arc on it and hit it deeper in the court) to allow you more time to get to the NVZ. If you are at the NVZ line, your opponents will not be able to hit to your feet as easily. This video will help.
If you are on the serve side you need to work on your transition zone game. This video will help.