You should have something like some chain and a turnbuckle pulling the bow down enough to stop it bouncing. Then, if you have the down points on the transom, use them to pull down at back. If Not, run a 50mm ratchet strap across the back and down to the trailer each side. Don't overdo it, those big straps can exert a lot of force. Just enough to hold it firmly.

 I normally put a block of pine between the motor leg(?) and the motor mount and tilt the motor as far down as possible. I drill holes thru the wood and cable tie it to the motor mount just so it doesn’t fall out if the motor bounces up. Then I also tie up the steering wheel to stop it the motor from moving (I also tie up the spair wheel and hub to stop it from spinning if you have one of them)

then as ranmar suggests I also use ratchet straps to hold the boat securely to the trailer with some rag between the strap and the hull to avoid rubbing. Finally I put cable ties thru the eye of the bolt on any u-bolts so they can’t rattle loose (have lost a safety chain before!)

I use the Yamaha tilt support for the motor. Takes the pressure off the rams

I chain either side of my boat down to the trailer. Just dont do them up too tight amd put undue atress on the hull!