For long distance towing these are pretty much mandatory. The transom tie downs secure your boat hull to the trailer and pull it down tight to the trailer bunks and keep it from bouncing on the trailer and stressing the hull, combined with a transom saver that takes (*some*) of the weight of the motor and transfers it to the boat trailer. These two items used together properly represent the very best way to tow your boat over long distances.
Ratchet Transom Tie-Downs
Transom Saver
(*some*) Some of the better brands of transom savers are spring loaded. You only want to have this item under a moderate load, meaning just enough to take the *some* of the weight of the outboard motor and transfer it to the trailer.
A few other tips, put the outboard motor in gear, either (forward or reverse) the properer should never be spinning from the wind pressure pushing against it. The inner seals are lubricated with gear case oil. The outer seals somewhat depend on water to help keep them lubricated. It might look cool as your going down the road with a spinning propeller but it's actually damaging the outer propeller shaft seal if it spins and is not getting water splashed on it once in a while. Don't forget to check tire pressure too, often overlooked. Long trips can really do a lot of damage to boat trailer tires if they are under inflated. Don't assume they should be at 35psi. On the side of the tire there should be information on what the correct tire pressure should be. On my trailers they are supposed to be at 50 psi and it always seems a little weird putting that much on them, but that is what they are supposed to have to keep them from over heating. Check the wheel bearings too, a few shots of fresh grease on them is always a good idea.