Training in soft sand is a sure-fire method for improving speed and first step quickness. You have to work harder to get your legs moving fast and it provides a great form of resistance. Training in sand while running uphill adds an even greater degree of difficulty.
To keep our athletes motivated and competing at DeLuca's we use mini-races, challenges and tag drills. The key is not to "overdo it." Cut back the volume and distance from your regular speed training program and you'll reap the rewards of "sand training."