I do not allow my dogs on the furniture. However, Scooby will get up there when I am not looking! At one time, I had four dogs and none of them dared got on the furniture except for the bed. I did make the mistake of allowing them to sleep with me. Which only did not work out because Scooby growled if I moved!
My technique has been to be "Alpha" Dog. They know I am "boss". They get on the furniture I yelled distinctly "Down". I also when petting them did not allow them to come up beside me. They always had to stay on the floor when petted. I also tell my guests to NOT allow my dogs on the couch. That may be the hardest people to train! I've had to yell at my guests to get the dog off the couch!
All of my dogs were adopted from the pound so they were NOT trained when I got them. I just always let them know that I am their master at all times. Believe me they remind me when a stranger approaches and they hide between my legs barking!!! Just let the dogs know their "pecking order" in the house and things should work out.
That and try aluminum foil... it works for cats...