I always tied my Nauty down from the swim platform brackets. If the platform was off the straps went over the small bracket left on the boat.
If you trailer does not have tie down spots, they actually make a strap that I believe bolts to the trailer and then retracts, as Robert said above.
Overtons You could also just wrap a strap under the trailer and hook onto the underside, if it is a channel type trailer.
A short story of why tie downs are important: I was always good about tying down my boat, but after going to one marina on the Delta, that we could not launch at, we were driving down the road a couple of miles and coming over a bridge, with a launch ramp built into it, my boat flew about 8 inches off the trailer and landed back on it. We immediatly pulled over and strapped the boat back down. Never again was it towed on the road without the rear tie downs on it.