Raw onions sliced and mixed with brown sugar. Drink the resulting liquid like medicine (this method has been around for a great many years and doesn't taste as bad as it sounds). You might need to leave the onions to marinade in the sugar overnight before enough liquid is produced.
Another method is the same as above, but cook the onions first.
Echinacea tablets reportedly help a lot and are used regularly by top athletes to avoid getting sick prior to important sporting events (just make sure they are very good quality tablets and not combined with other substances/vitamins).