"Spider veins" are an extremely common problem.  They are thin, small bluish or purple veins that tend to occur in crops on the legs. They are caused by such factors as genetics, hormonal factors (estrogen and progesterone), and lifestyle factors.  Prolonged standing or sitting causes blood to pool in the lower leg veins, causing them to become dilated or "stretched".  This pooling causes new veins to form as the blood looks for new ways to make its way back to the heart.

The best thing you can do to prevent leg veins from developing or worsening is to wear compression stockings.  These tight stockings encourage blood to return to the heart rather than pooling or creating new veins.  Wearing compression stockings also makes sclerotherapy more effective.  Over-the-counter support stockings can be purchased at many pharmacies.  

Most experts agree that sclerotherapy is the most effective treatment for most spider veins.  This treatment involves the injection of a "sclerosing agent", most commonly a sugar or salt solution, into the veins with a very fine needle.  This encourages the veins to close up, improving the cosmetic appearance of the area.