Just read this at Gawker.

Here's how it's done, my fellow laptop hobos: You order something, you tip. If you sit for more than 90 minutes, you order something else, you tip again. Repeat as necessary, tipping each and every time. If the coffee shop you're in is filling up, invite someone who's looking for a seat to join you. (This doesn't require conversation—shudder—just a pre-cleared spot at your table and a nice, polite, silent nod.) If the place is really getting packed and you've been there for a while, pack up your shit and leave. It won't kill you to unplug and go for a walk. And you know what? There's another coffee shop somewhere nearby that isn't packed, maybe even one that's empty, a coffee shop where the baristas will be happy to see you and your laptop. Go find it.