The flatbed tow guys use straps over the tires (kind of a short net with a ratchet strap). In other words, they strap the wheels down and let the car ride on the suspension. To me, this seems like a better idea than trying to compress the springs by strapping down the frame or the suspension parts — it's less likely to break something, and it doesn't shift if the trailer bounces over a bump. Is there a reason why you don't do that?