Crunch a clove of garlic and put it on the bottom of a mug. Warm some milk and when hot (but not boiling) pour over the garlic. Drink as hot as possible, hot drinks are always more effective. Try not to think too much about it while you drink it.
I assure you once is enough to take care of an oncoming flu. I don't know about colds.... ooops... my cold is getting better just at the thought of it! 
