What Tom & Bob said.

It varies depending on size, weight and overall contour of the models belly. Whether there's any ventral fins or scale danglies underneath can dictate how some planes need to be carried.

The type of trailer can make a big difference. Big trailers with torsion bar suspensions and multiple axles can ride so nice, you could almost just sit your stuff down and it won't move. I wouldn't - but you get the point. If your trailer's small and bouncy it'll change your tiedown needs.

A lot of guys lay sheet foam on the trailer shelves. Either use bungees and foam blocks hooked to screw-down anchor points, d-rings, etc. or cover the shelves in velcro loop fabric and use velcro hook straps. Velcro option works well but is more expensive. I do both by laying sheet foam on top of velcro loop covered shelving. I velcro the small stuff in place, and use anchor points and bungees on bigger aircraft.

Once you place a model on its belly onto sheet foam, the foam will provide very good forward and aft restraint by itself because its got a lot of grip, but the model still needs to be secured with straps and/or bungees.
Try not to transport jets on stands or on the landing gear as they become fulcrum points over rough roads, and can cause stress cracks unless very carefully placed and/or contour fitted to the shape of the fuse. Even then, I don't like to transport in/on any kind of stand. I've seen a Bandit cracked nearly in half that was transported strapped to a field assembly stand.

If the model is very large, you can make a custom width wood cradle with a plywood floor. Foam line the bottom, place the model on its belly then attach 2x4 supports at the wing tube points. Secure the wing tube to the supports with cinch or velcro straps. Then screw the wood floor of the cradle down to the shelving or trailer floor. Remove the screws and slide the whole shebang out of the trailer once your there. This method is a bit of a PITA, but its also about as bulletproof as it gets.