Don't cross your legs when sitting.
Exercise regularly. Walking is a good choice. It improves leg and vein strength.
Avoid weight gain.
Avoid standing for prolonged periods of time. If your job or hobby require you to stand, shift your weight from one leg to the other every few minutes.
Wear elastic support stockings.
Don't wear clothing or undergarments that are tight or constrict your waist, groin or legs.
Eat high-fiber foods like bran cereals, whole grain breads, and fresh fruits and vegetables to promote regularity. (Constipation contributes to varicose veins due to abdominal straining).
To prevent swelling, cut your salt intake.
Exercise your legs. This can be undertaken even if you have a sedentary desk job. (From a sitting position, rotate your feet at the ankles, turning them first clockwise, then counterclockwise, using a circular motion. Next, extend your legs forward and point your toes to the ceiling, then to the floor. Then, lift your feet off the floor and gently bend your legs back and forth at the knees).
Elevate your legs when resting.
Get up and move about every 35 to 45 minutes when traveling by air or even when sitting in an all day conference. (Opt for an aisle seat in such situations).
Stop and take short walks at least every 45 minutes when taking long car rides or limited exercise of your legs as advised previously.