Guaifenesin
"Disclosure: This is just one person's experience. As such It does not have any scientific validity. However, I hope some scientists will consider some actual research on this. 
     I have been taking Guaifenesin in 400 or 600 mg tablets at the first sign of a cold, and my frequency and duration of colds has decreased from several a year to one or less.  I am 68 years old, and in general good health.  Your results may not be the same.  Remember, one person's claims/results are pretty close to meaningless from a scientific research viewpoint."