I am not sure if there is anything you can do. I have the same problem. I know if you drink too much alcohol, smoke (a vaso-constictor), or are dehydrated your veins become thin. I have heard that niacin can help but when I took it I had to many hot flashes, so I stopped. Hope someone else has insight on this.
I believe it is true because all of my veins are huge. However I was injured and lazy for two years and gained weight and the veins were no longer visible. When I hit the gym hard again they began to appear. Do a lot of arm curls and you should see results within two months. I lift 45 - 50 pounds per arm.
Davis makes a great point! When I used to lift I had great veins...then I got lazy too. Exercise increases blood flow which, in turn, enlarges the veins. Damn I really should start lifting again!