I take a super dose of Vitamin C (3g) if I ever feel like I'm going to fall sick. It's amazing how well that works and keeps the flu at bay. I make sure I don't take it daily though - prefer the natural route - fresh fruits

I used to take protein powder, but I think its a waste of money if you are not trying to seriously bulk up. For the ones who want lean muscles, enough eggs and chicken breasts do the trick.