Pets On or Off the Furniture?

Our dog Otto doesn't usually try to get up on the couch while we're around, but as soon as we leave the room, he's all over it. (His full-sized dog bed is just a few steps away, but he usually prefers to lounge near his favorite piece of furniture with his blanket and pillow.) Since we adopted him, we've found yet another use for our Prince Aha Stool - keeping him off the sofa...

If he was a little lap dog, we'd probably invite him up, but we don't want our dig-happy greyhound destroying our couch. What about the other pet owners out there?

Do you let your pets on the furniture or keep them on the floor?
If the sofa is off limits, how do you keep it that way?