This is usually seen with lower body fat levels and decreased sub-cutaneous water retention.Density,distribution and amount of veins could be a genetic thing,as some competitors who are 4-6% during contest time have several visible ones running through the arms, where as another could have massive prominent bundles running rampant ie.Paul Dillet
Drugs which increase red blood cell count contribute to extra prominence and girth of the veins, such as EQ as mentioned by slobberknocker,as well as EPO or it's improved version Aranesp(Darbepoeitin Alfa) which is more effective at increasing hemoglobin levels and also has an increased half life of 3 fold.
Oxymethelone may show it's true rbc boosting effects on vascularity once sub-Q estrogen is removed as it is indicated for refractory anemia,aplastic and faconi's.
Some swear by the supplement Niacin, as it is a vasodilator, but must have some bp maintaining effect as well because vaodilators usually decrease blood pressure, which I noticed causes a reduction in vascularity. ie. Viagara and minoxidil.
Cheers...B32