My dog sneaks onto the couch!

Is the couch totally off-limits to him? Or do you allow him up at certain times?

If it's totally off-limits:

Prevent him from being able to get up on the couch when you are not present. He's already learned that it's safe to be on the couch when you're not in the room, so you need to stop allowing him to be there in your absence if you don't want him up there. Use a baby gate, close a door, put your dog in another room or its kennel, etc.

Some people say to place something on the couch so your dog can't or won't get on it, but it's not always successful, and even if it it it's annoying to have to rearrange your room each time you come/go from the room. It's easier to simply make sure he can't get it on it when you aren't there.

Let him know he's doing the right thing (praise and/or food treat) by laying on his own bed, and redirect him to his bed if you see him lying on the couch. Provide good toys on his own bed, that he can only have while laying on his bed - stuffed Kongs, a good chewbone, puzzle toys, etc.

 Kim's advice is spot on - a soft bed in the same room as the couch would give your dog the option to have a place of his own to rest that is soft and comfy.  The more you reinforce him for going to the bed, the more he will go to the bed;-)