To be honest, if you have **** veins I wouldn't bother, it just makes it more unpleasant for you. In extremis you could use veins in your feet, ankles or groin, but I wouldn't want to be playing around with those just for blood donation.
A few things to improve your vein visibility (some of which people have already said):
1) Make sure you're well-hydrated. Drink a pint or two of water before you go to donate (but not so much that you will desperately need the loo whilst hooked up!)
2) Stay warm on the way to the session - wear lots of layers. If you're cold, your body will close blood vessels near the surface.
3) Get a stress ball, or something that exercises your forearms and do a few reps to make your veins stand out.
If this doesn't work, aside from going in through the groin, I'm not sure there's much you can do.
Some people just have hard veins to find. I see it all the time on people who have to give blood samples on placement.
They couldn't find my vein to begin either, well they did just couldn't get any blood. Apparently I wasn't relaxing my arm enough because I was scared due to it being my first time.