Say, "I'll tip at the end"
Yeah, we know you will. You'll order expensive, complex drinks all night under the pretense of, "Don't worry, I'll take care of you before I leave." Then you'll order a $3 beer, leave a buck, and call it good.
Complain that your 3/4-finished drink is wrong
Did you seriously not realize that your Manhattan was actually a White Russian until it was almost gone? Usually, a bartender will gladly rectify a mistake. This is not that time.