Zicam zinc nasal spray (actually a gel) works great on colds ! Need to start using it as soon as possible for best results. Available at most stores.
For the flu, Oscillococcinum is fairly effective, but again, need to start on it ASAP (within first 24 hrs. or so). This you will have to get from a specialty store like GNC, Wild Oats, Vitamins Express, etc.
Echinacea is a good preventative as it will boost your immune system. But you need a good pure source (liquid form is best, and make sure it is glycerin-based, not alcohol-based; a few drops in a minute amount of spring water to get it down). You also need to use it correctly, as you can overdue it and that is counterproductive, ie. overstimulating your immune system can have some nasty side effects. Best to use it in cycles, say 3 weeks on 3 weeks off.
Finally, if you have inhalation exposure via someone's coughing/hacking/etc., nasal douching (warm salt water will do the trick) ASAP after the exposure has been proven to be very effective, some tests have shown up to 70% effective.