i let my dogs on the couch also. i think it depends on the size of the dog. i have a cocker pup and toy poodle mix.
anyways, everytime zoe gets on the couch gently push her off and say "OFF". (or any other word you'd like to use) if she gets off and goes to do something else, praise her. you have to be consistant EVERY time. if you let her sleep on the couch only a couple times all the training you've done will just been ruined. make sure she has a bed. Good Luck!