What can I do to prevent them?
There's no way completely to prevent varicose veins. But improving your circulation and muscle tone may reduce your risk of developing varicose veins or getting additional ones. Walking is much better for the veins and helps the blood return to the heart from the legs. Things which may help include exercise, watching your weight, eating a healthy diet, and avoiding high heels. Elevating your legs, changing your sitting or standing position regularly may also help.
In occupations that require extended periods of standing then a few steps should be taken at regular intervals to help circulation. No creams or drugs are available to prevent varicose veins. The earlier varicose veins and thread veins are treated the better the long term and cosmetic outcome.