Compliment the music
You may not love My Bloody Valentine, dissonant jazz, or Brian Eno's early ambient work, but you should damn well pretend you do. It shows that you're appreciating the environment, and nothing tickles a barista's pleasure center quite like a compliment on their obscure musical taste.
Keep buying things and tip again
One cup of coffee is fine for starters. But once your mug has been empty for an hour, buy something else. And it doesn't hurt to own up to the fact that you're essentially purchasing a few more hours of Internet. Why do you think coffee shops always have banana bread by the counter? It's not because anyone likes it, it's because people feel guilty and need a way to spend another $2.
Dan Gentile is a staff writer on the National Food and Drink team at Thrillist. During the course of writing this article, he was actually witness to a customer aggressively complaining about there being no seats in the coffee shop. Follow him to salty looks at @Dannosphere.