For me it's buttermilk. I was raised on it, so I'm accustomed to it, but it ain't for everyone.

Usually a good low fat fruit flavored yogurt will do just as well. Watch the sugar content. The milk proteins and the enzymes in yogurt or buttermilk have a mildly sedative effect, and for those who aren't lactose intolerent, it is fairly easy on the digestion.

For those who are lactose intolerent, a bowl of seasoned rice or some toast might be enough.