As soon as you feel yourself getting weary, take some Emergen-C (available at GNC, Trader Joe's, and other healthy/hippie places). It won't cure the worst, but it'll help. I was introduced to it in the world of opera, where grown adults treat illness like the boogeyman.
A multivitamin should be a habit - but at least try to have one daily when in production. Rest, healthy eating, and lots of water should be everyday priorities. It's hard to keep up when you're working like crazy on a show, but this is really when you need it most.