Well, throughout my life, a simple common cold would often induce severe asthma reactions (sometimes requiring emergency hospitalization).
Also, each winter season, I would frequently average about 4 to 5 colds/flu's.
So as you can imagine, I've been strongly motivated to find a "cure"!
I've read extensively on this topic (both scientific/anecdotal), and there are only 2 things that have worked for me:
1) Vitamin-D.
During regular everyday life, I take 2000 to 4000 iu per day. (So that's 2 to 4 average pills per day).
But... if someone around me has a common cold (rhino virus), or flu (influenza virus), or I feel the sometimes starting... then I IMMEDIATELY up my dosage to about 6000 - 8000 iu per day (6 to 8 pills per day).
Since doing this my hospitalization rate has dropped to ZERO.
I have even gone for periods of up to 2 years without exhibiting any significant flu/cold symptoms!
And when I do get a cold/flu the impact is much less severe.
SIDENOTE:
In addition, after about 1.5 to 2 years of taking these doses of Vitamin-D, my severe summer allergies also dramatically dropped in intensity.
Likewise, my girlfriend's severe cat allergy also vanished after 2 years of taking these doses.
I realize this is HIGHLY anecdotal... and hardly scientific... but the dramatic effects of long term Vitamin-D intake was quite surprising to me.
ALSO NOTE: I make sure to buy one of the premium Vitamin-D product, that has the best ratings/lab results. In Canada that tends to be the Jamieson brand.
So if you are going to attempt this experiment on yourself, you may as well spring for one of the better lab-producers of Vitamin-D in your area.
2) Don't rub your eyes or nose, when using public transit!
The rhino (cold) virus, usually infects the body when you rub your eyes or nose with your hand... after you hand has touched a surface infected by someone with the common cold.
So if you take public transit... don't touch your eye/nose area with your bare hand, until you have washed your hands.
This can go a LONG WAY to reducing the number of colds per year you experience.
NOTE: Unfortunately however, the flu-virus is a different matter, and is spread through airborn inhalation.