Drink lots of water. The more water you can stand, the better. It really will lighten them over time.
Allergies are a cause for dark circles. Every morning I take a decongestant.
If you're going to be in the sun for a long time, use sunscreen. The sun will just make them ten time worse.
Try and avoid caffeine. I know it can be hard, but I've heard this helps.
I can't think of much else besides don't rub them... I'd recommend some cosmetic products, but I don't think that's what you were looking for..