Trailer home screw-downs. I have a set that are heavy duty and about 3.5 feet long with a spiral on one end. Once those are in the sand minus 4 inches, they hold great. Other ideas are to dig a trench and bury a 4' long 2x4 that you tied to in the middle. I've also seen 5 gallon buckets buried but I wouldn't trust the handles...drill through opposing lips and tie to those.