Our boat tended to slide from side to side at the rear of the trailer, too. My concern was that this motion appeared to be stopped by the trailer bunk hitting the boat, on the inside of the chine. You could put all the tension you wanted on the tie down strap, and the stern of the boat still slid back and forth as you turned while trailering. The easy way to see if this is happening is to do a very low speed turn on a parking lot, and then feel under the boat on the side which was the inside of the turn, to see if the boat is up against the side of the bunk. Then do another turn in the opposite direction, and see if the boat shifts toward the other side of the trailer.
Our solution was to move two unused tie down anchors from the brake light units, down to the inside axle u-bolt brackets, and use not only the main tie down strap to keep the boat on the trailer, but also, two shorter tie down straps between the transom u-bolts and the relocated tie down anchors, to keep it centered. Locate the anchors wherever you want, so long as each strap is capable of applying a lateral force to the boat. To do this, the straps have to either "wrap" slightly to the inside, or come laterally away from the boat at a fairly steep angle. Otherwise, you're tending to pull the boat toward the front of the trailer, or just hold it down. Neither is a particularly effective means of eliminating lateral motion.
It takes a little adjusting to get the boat centered on the trailer. For transom straps which "wrap" around the boat, go ahead and tension the main strap, and then put moderate tension on the transom strap on the side toward which you want the boat to move, and nominal tension on the other transom strap. Then, pull the trailer in a 360 degree turn (heck, 180 would probably work just as well), and let the boat slide out, onto the higher tensioned transom strap. If the boat's then centered, retension the other transom strap, and you're set to go. If not, do a little more fine tuning. If your transom straps go laterally away from the boat, rather than slightly wrapping, just reverse the tensioning procedure. Either way, the objective is to have the boat tension one of the straps as it centers on the trailer, and then manually tension the other strap.
We got straps which have a soft plastic pad under the buckle, and a velcro closure which wraps around the buckle and strap, when the buckle is closed. Both features are recommended. There's not much strap material left over, so depending on your trailer, the size of your boat, and how it rides on the trailer, you might need more than 4 feet.
