The Importance of diet during illnesses is seldom taken into consideration. Yet, its what you eat, that can hasten or slow down your recovery process.
With regard to the Common cold, Eat only food that increases heat in your body. A Few pointers which i found useful were:
1) Avoid yogurt, or any cold things, like ice cream, and soft drinks which are refrigerated.
2) Drink at least two glasses of a mixture which is as follows : 1 glass of the mixture contains half a glass of water and half a glass of milk. This combination is esp useful when one has infections and not just allergies. This makes it light on ur digestive system and kind of gives u some much needed heat in the body. Also, add some pepper and a pinch of tumeric in the mixture if you can.
3) Eat some pulses if you can. Dont eat potatoes as potatoes take away heat from the body.
4) Avoid Watermelon, any citrus fruits. Fruits either worsen one's cold, or kind of have a neutral effect, So its better you dont go near citrus fruits at all.
Apart from Diet, one more thing is to cover your ears effectively while sleeping. Sleep sideways, so that one ear is pressed to the other pillow, and you can cover the other ear by pulling your rug/blanket over it. This makes it easy to breathe too.
Well, All these measures are esp useful when one has infection, rather than just allergy.
Hope it helps.