things that will help your body in addition to Jeckyl's recommendation food, water, multivitamins is I would actually add along that lines, that vitamins aren't necessarily going to do everything you expect them to do. I would go for supplements if you're going to go as far as to stop by a convenience store and get something OTC like that to assist with the hang over. I would recommend iron and magnesium, b12, vitamin c (allows uptake of supplements to be more effective) and b6. believe it or not, sodium and potassium will also help flush out your system- so eat something salty, and then a banana. restore electrolytes with gatorade. and keep flushing fluids through you, if you want to get fancy with it, a water with balanced pH is best for cleansing a system- like VOSS or Fiji. But any water will do to flush you out. Dehydration is the main reason hangovers suck so much, and if you hydrate prior to a night of drinking, and afterward before going to bed- it will be likely to lessen the effects of the hang over.
Orange juice and water the next morning is what you want to go for, after a night of heavy drinking. Or just pop a vitamin D supplement if you hate orange juice. Eat something high protein, with greens on it, and some carbs. Like a good sandwich- or perhaps a soup. Once again, don't forget to have yourself being checked up on regularly.