You know how at parties, people always confuse whose drink belongs to whom? In the end, you have to toss out unclaimed drinks. Well, I hate waste. As soon as I saw the Wine Lines from See Fred, I had to have them (we got the Vintage version). They're great because now everyone can identify their glass. What's not-so-great is the print started rubbing off after the first time we used them, and they only work for wine glasses.
See Fred also says Beer Bands, but today I found these clingy labels from Betsybaloo, that would work just as well. All options are reusable - you just have to remind guests not to toss them, if you happen to be using disposable cups.