Colic is a very serious intestinal disorder that can seriously hurt your horse. There are several things you can do to prevent colic. Provide drinkable water at all times as well as a constant source of quality roughage. Do not feed an excessive amount of grain.
Prevent sudden diet changes (any necessary diet change should be gradual); one of the most common and well-described causes of mild colic is the diet. Many horses that develop mild colic are being fed too much grain (for the amount of exercise that they are getting).
For more information, please read the story Colic - How to Deal With It.