Do not park between buildings, where wind tunnels can develop. Remove electrical equipment. Use wooden blocks at the trailer's wheels. Deflate the tires. Tie the boat and the trailer down to something secure.
Ground anchors are best and can be purchased if there is time.
If you don't have a trailer, fill the boat with water and tie it to the most secure thing you can find in your yard. If the boat is very small, turn it upside down and lash it to the ground.