From DrWeil.com

Today's Tip: Preventing Gallstones
Gallstones are composed of cholesterol that collects in the gallbladder and crystallizes, forming lumps or "stones." They can range in size from a grain of salt to as big as a golf ball. If they remain in the gallbladder, they are usually benign, but if they begin to move out of the organ they can cause severe nausea, vomiting and pain that can last for hours. Here are some tips for avoiding gallstones:
Adjust the fat in your diet. Too little fat can promote the formation of gallstones; if you already have stones, too much fat may trigger an attack. Aim to make fat intake around 30 percent of your daily calories.
Supplement with vitamins C and E. These vitamins help reduce the cholesterol levels in bile, and research suggests that they can also retard gallstone formation.
Increase your activity level. Try for at least 30 minutes of physical activity every day. The risk of gallstones is increased by a sedentary lifestyle - fair warning for those of us who spend a lot of time sitting.