Astragulus is a great remedy for colds, as is colloidal silver and homeopathic cold and flu formulas.
I personally find echinacea only works best at the very first sign of a cold... once the cold is full-set I find the remedies listed above work better. Raw garlic is another good one, as is fresh ginger, thai curries, coconut milk, and fresh chiles (I usually go to a local Thai resteraunt and get some Thai green chili soup~ it's made w/ vegis, fresh ginger, green chilis, thai curry, and coconut milk... it makes your nose run like crazy but chases the cold out!)
Don't forget the vitamin C as well!!