Keep the tires (on both the tow vehicle and the trailer) correctly inflated. Soft tires will allow the tow vehicle (and the trailer) more side to side movement. Over inflation can also be a problem, causing the trailer to bounce excessively. Also, light truck tires are stiffer than passenger car tires, and therefore may reduce the likelihood of sway.
Use all the same type and rating of tires on the trailer. A significant difference in the rolling resistance or handling between the two sides of the trailer can contribute to sway.
Always load you trailer evenly, side to side as well as front to back. Do not carry a load that can easily shift from side to side (like a half full water tank). See article on correct loading of your trailer. Check your tires before every trip for uneven wear, and maintain the air pressure at or near the rating if you are loading the trailer to the max weight. While you are checking the tires, get in the habit of checking the lug nuts. Maintain 10% - 15% of your trailer weight on the tongue, but keep an eye on the tongue weight rating of your trailer.