Remove the trailer tires and store inside (consider doing this for theft, too). You can block the trailer. This takes the load off the tires.
If the tires have to remain on the trailer and you can’t park on concrete or asphalt, position the trailer over plywood if at all possible. Parking on grass increases moisture that not only harms the tires but, over a long period of time, eventually affects the trailer frame. If it’s possible, move the trailer a few feet every month or so to rotate the tires so as to avoid flat spots.
Increase the jackstand height to remove excess rainwater and melting snow.
Don’t park under trees.
Apply some WD-40 or a spray lubricant on the rollers, winch gears and electrical connections.
If you have a galvanized steel trailer, do an inspection for any rust spots. If any are found, now is the time to sand, prime, and paint. Don’t let corrosion get to the point where the trailer is unsafe.
If your trailer is parked with the tongue facing the street, a thief can easily back a tow vehicle to the trailer, hook it up, and be off within a minute. Instead, park the trailer so that the tongue faces the garage or away from the street.
If your boat a small, many trailers are built with swing tongues that can make storage in a garage a possibility.
Put a lock on the trailer tongue.
Consider removing the engine. Take all valuables and electronics off the boat.
Have a friend stop by from time to time to inspect the boat and trailer.