This graphic, posted on personal trainer Lucas James' blog details the relationship between alcohol and gaining weight. The empty calories in alcohol add up fast when you consider alcohol has nearly as many calories per gram as pure fat. A frozen margarita is similar in calories to a cheeseburger and drinking a glass of wine is like eating a slice of cake, calories-wise.
Besides drinking in moderation, the graphic reminds us to use better mixers (e.g., tonic water) than the usual sugary drinks. That is, if you want to get drunk, not fat.