Aldo is correct- a friction anti-sway device is very effective. There are a couple of things to consider, though.

The Al-KO clamping hitch is an excellent piece of gear, BUT WILL ONLY WORK WITH A 50 MM BALL! Normal 1 7/8 or 2 inch balls are too small or too large for the clamp to handle. I have installed a few of these on Cobra trailers and the results are excellent. Note that the ball MUST be welded or otherwise fixed to the receiver extension to keep the clamp from loosening the nut on the ball shank. I machine a "flat" on the base of the ball and weld a piece of bar stock to the hitch to keep things from moving.

This is a rather expensive solution, and the AL-KO hitch is not available in the US, so I bought some from a supplier in the UK. Note that this design is the only one I have seen that will work with the usual European surge brake coupler.

For a tongue without a surge brake, there are several designs that are inexpensive and very effective. Most will require some welding and proper alignment is necessary, so a pro should be consulted. Here is a link to sway control devices from etrailer.com: http://accessories.etrailer.com/search?w=sway%20control

Mark Mocho
MM Fabrication