In addition to the winch, I have a nice wide ratchet strap going from the trailer to the bow eye at an angle of about 30° - not so much to keep it on the trailer but to stop (or at least slow) the boat in the event of an accident that brings things to a sudden stop. The boat might be blunt and full of air but I don't want to see in my rear-view mirror the boat coming through the rear window, or watching it slide down the road into oncoming traffic or pedestrians.
I strategically replaced some of the nuts on my trailer with galvanised eye-nuts (or what ever they are called) and lots of threadlock. The tie-down straps at the stern are point-to-point to the boat with no scope for movement. I still check and, if necessary, tighten the ratchets a couple of clicks after a couple of miles though.
The boat is securely fixed to the trailer but other than driving very slowly and avoiding potholes where possible there's not much than can be done about the trailer bouncing around on the rubbish roads round here.
"Can ye model it? For if ye can, ye understand it, and if ye canna, ye dinna!" - Lord kelvin