Garlic and honey! I've heard of people crushing up a clove of garlic and just taking it in a teaspoon of honey- gross, but effective, I hear. And as others have said, zinc, vitamin D, vitamin C, and I've likewise heard fabulous things about elderberry (and I've heard it's safe in pregnancy, but never looked it up). And sleep- sleep sleep sleep (easier said then done, I know!)
I started feeling a cold coming on so I doubled my vitamin D, added in some vitamin C, and made DH get up with Nigella on all his days off so I could catch up on some MUCH needed sleep. It never materialized, which is awesome. I was stressing about being sick right before my due date as well.