What are the best products for keeping your hair clean without showering?

I'm spending a good portion of the summer at a marine lab on an island, and to conserve freshwater, we only shower once or twice a week. I hate having dirty, greasy hair! I've heard of dry shampoos and such, but which ones work best and last longest? I have thin(ish), straight hair, so I need something that will give me a light, clean feeling with no residue. If you have a suggestion, please mention if you've used the product before, how long you used it, and what type of hair you have.