Feed quality mix grass hay. Don't feed sweet feed. Don't overfeed feed pellets. Don't feed any mix with corn or wheat in it. Keep your horses feet in good condition, including regular trimming/shoeings (every 4-8 weeks). Keep your horses fit (don't let them get overweight).

Here's a good article on the subject. Now, I'm not a barefoot-only cultist, but this article does provide a lot of good information on feeding horses and laminitis.
http://hoofrehab.com/end_of_white_li...nitis%20update