You can keep your veins in good shape if you let a spot heal before you hit on it again. This clot could get stuck somewhere else in your body, including your brain. This is really dangerous. You let one spot heal while you use another. A vein takes at least a couple days to heal.
More time is better.
Shooting downstream from your last hit means you won't be pushing any blood clots into your bloodstream. It may be awkward, but it gives you more options. This makes finding a vein A LOT easier. Rotating your spot is recommended, but sometimes you may not have the veins, or the time to find one. The veins in your hand look tempting, but if you're not in a hurry, then save them for when you need them.