I'll talk about a homebuilt trailer with leaf springs. Towed OK but I got pretty good sway when a semi passed, oddly most noticeably when it passed in the same directly.

After doing all of the assorted things mentioned above by others (which did help some) I ended up installing a sway bar kit which significantly corrected the situation. Your mileage may vary.

See http://www.etrailer.com/Weight-Distr...ies/83660.html

- John