First, try to avoid being coughed on by someone who is already sick! Wash your hands often, but don't use those anti-bacterial soaps, which just make the problem worse in the long run. Get fresh air every day, both by going outside and by getting some fresh air into your home and workspace. If possible, limit the number of people you come into contact with who are sick -- encourage your friends not to visit when they have a cold. Do some sort of exercise that strengthens your lungs -- walking, jogging, swimming, yoga. Get enough rest and eat well. Cook with onions and garlic often during the winter months. Eat lots of dark green, leafy vegetables like kale and collards.
Astragalus is a wonderful herb, and acts as a tonic to strengthen and build the immune system. In my house, we take a tincture daily from Autumn until Spring. I also add a few pieces of Astragalus root to each pot of soup I make. I also love the herb Ashwaganda, which can be made into a chai-like beverage. Tibetan herbalists say that if you drink Ashwaganda tea every day for a year, you'll have the strength of a horse for the rest of your life!
If you do feel yourself coming down with a cold or know that you've been exposed, Echinacea at the first sign may help to ward it off. And, if you do end up with a cold, Elderberry syrup can go a long way to relieving the symptoms, and it has anti-viral properties that fight the virus as well. I've recently been experimenting with Usnea, a lichen, which also has anti-viral, anti-bacterial, and anti-fungal properties. It seems to have a good effect in fighting colds.
I find that Goldenseal is quite amazing in dealing with sinus infections, which can be difficult to treat since there is very little blood flow to the sinus cavity. However, Goldenseal is an rare plant, difficult to cultivate, and should only be used when absolutely necessary.