When we go fast or have small step sizes (16-5 or smaller) we almost straight leg it (spiking for REALLY small steps). For the larger step sizes at higher speeds we really talk a lot about keep their hips back and not letting the pelvis go forward (this means watching their upper body posture). If they lean back and try to go forwards the leg is naturally going to be late, and will also cause the marching technique look different marcher to marcher….so then sets and spacing always look inconsistent. Keeping those stomach muscles engaged keeps them taller, and keep the body weight going forward, and not reacting to their legs moving first.
Make sure their hips stay back on the first few steps and that should help give them a chance to stay in step most of the move. They will get tired at the ends of moves and musical phrases, so stress they have to finish off a 16 count move visually as well as musically.
It has helped our band…others can probably say it better or tell me I’m wrong, and that’s fine. Just our $.02.