Mobile home anchor. Two of them. One at the mast, one at the aft crossbar at center. Use webbing tiedown straps.
See pic:
Dig a hole about half or so the height. Then auger the thing in until just the bracket is just above the ground. Fill the hole back in and stamp the dirt down. It's nice and tidy if you can take a bit of PVC piping and hammer it into the ground so as to surround the bracket bit - keeps the sand from the bracket and peoples' toes a bit safer. If the area is regularly mowed, the real nice thing is to have the bracket a bit below ground level, the PVC at ground level so it can be mowed over.
This very setup kept my boat unharmed when a: every other boat on the beach flew off to Kingdom Come, and b:one of these boats (was on a trailer) flew into my Ford Ranger, the whole thing flipped my truck upside down.
It should be said that in the above example, the boats' anchors held. AFAIK, most all the boats were tied with some cheap line and the line failed, except of course for the boat that tangled with my truck. That one was on a trailer, which is not a good way to keep a boat on a beach.