I've rigged, fitted, towed, and modified a bunch of trailers. These are the things I have done to mitigate trailer sway (in order of effectiveness):

1. Longer trailer tongue. Longer is always better at reducing sway. Unfortunately it also increases trailer overall length, tripping hazard, etc.

2. Mass and CG management for trailer. Move weight out of the tail, but maintain modest tongue weight (~5-10% of trailer mass). A single-seater trailer shouldn't weigh more than about 900 lbs.

3. Stiffer tow car rear springs and shocks.

4. Stiffer tow car rear tires.

5. Stiffer trailer tires.

Things that are generally more trouble than they're worth:

* Supplemental sway dampers--They're a pain in the ass bandaid for other trailer issues, and you can't depend on random crews to rig them properly.

* Tongue weight over 10% of trailer mass--Deteriorates car handling more than it improves trailer handling, and makes your trailer a pain to rig and tow.

* Dual trailer axles--Maybe OK for a two-seater, but generally makes your trailer a pain to handle without a tow car.

--Bob K.