I wish I had more answers for you but I looked on google and found this:
http://www.nativeremedies.com/ailment/immune-system-boosters-info.html
maybe some of the ideas there can help.
I think with something like a sinus infection there really isn't much you can do to fix it besides take something. Have you tried a netti pot? I have heard vitamin c doesn't have any real effect on you getting better, it only helps before you get sick.