I saw, heard or read this somewhere recently. If you go to the bathroom and/or leave your drink, toss it and order a new one when you return. I don't frequent bars but if one of my girls ever does, I plan to pass on this advice. In my day no one ever thought about this kind of plan.
A few years back, I would go out with a friend of mine from high school. Since she wasn't paying attention to either of our drinks, I would take mine into the restroom with me. I'm not much of a bar person anyway. Men need to exercise caution too.