I swear by pro-biotics. Have you tried Danactive?
Teaching kids in judo (who wipe their snotty noses on their gi sleeves-- and then you must grab the gi sleeve to throw them-- I know-- ewww!) I used to get tons of colds and respiratory illnesses.
But, since starting Danactive, I haven't caught a single cold (yet). It's touted to boost your immune system. I believe it.
It's difficult to find, but if you have a Whole Foods near you, you're in luck.
Here's a link.
http://www.danactive.com/
Also-- do you wash your hands often? Remember.. just touching a door handle (especially on your way OUT of a public bathroom) or a community phone ior fax machine s enough to get you sick. Be vigilent about keeping your hands away from your face-- and be sure to use alcohol wipes.. alot.