Differnt things work for different crew - I always recommend Green Tea (get it from your Tea and Coffee box if doing Japan flights) or easily purchased at your supermarket.... Also recommend lots of room temp/tepid water with lemon in, or as mentioned vitamin c, ecinacea or multivitamin will help you deal with the usual common colds.....
But in all honesty, and I know you dont want to hear this - EXPERIENCE will be the most helpful thing in fighting common colds/flu!!! Over time, your immune system will become stronger and you will find it easier to fight the basic common cold etc
Good Luck!!!
(oh PS I find Jack Daniels is very effective in fighting the usual illnesses - maybe because I am an alcoholic, who knows? 
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