Like everything your dog does, it must be earned. The rules for sitting on the couch:
Dogs should earn the privilege of getting on your furniture by at least, doing a sit. This should be followed by a command to get them on the sofa, like the command Up. After your dogs perform a sit, simply pat the couch and say Up. They can only get on your couch on YOUR terms.
You should teach your dogs a relocation cue (another place to go other than the sofa) and train this command frequently. Examples would be, “go to your bed,” or once off the couch just place your dog in a down by your feet in front of the couch. This teaches him that you can let him up, BUT, you can also tell him to get off and go somewhere else. If your dog has a tendency or potential to guard the sofa, doing this exercise frequently would help to minimize any resource guarding that could develop.
Dogs with strong leader-type personalities may have a greater tendency to guard “their” space. The stronger this tendency, the more I would tend to limit time on the furniture.