Smart tie down
Best way is through the wheels as others have recommended. I used 1,000# ratchet straps through the rear wheels for and aft. Let the car suspension do the work along with the trailer suspension.
No longer own the rig or the Smart, but it worked great while we owned it. Towed it thousands of miles without a problem.
Rick and Lynda Smith
2004 NEWMAR DSDP M-4011-SPTN 8.9 Liter Cummins
2011 GMC SLT 1500 4WD CREW toad