To prevent spider and vericose veins, you can do more sports and refrain from standing for too long at any one time. If your desk job requires you to sit for long hours, stretch your legs and rotate your ankles. Avoid wearing heels too often or jeans that are too tight.
These veins form due to improper and irregular blood flow so regular exercise and movement of the areas that are prone to them will help to prevent them from forming.
P.S. Sorry about the absence of a picture in this entry. Spider and vericose veins are just too scary to look at.