Dogs don't generalize well (if at all), so if you teach them not to get on the couch while you are there, that's exactly what they learn - don't get on the couch when you're there. At night, when you're asleep, is a whole other ballgame.
Do they have dog beds they can sleep on? Reward them for laying on them.
Otherwise, I'd put something (pillows, laundry baskets) that will prevent them from getting on the furniture.