Well, that's tricky since most pets like to sit on couches and chairs. This is one battle I've never tried to fight with any of mine.
I did see a product advertised a while back, some kind of "mat" that would actually shock the animal a little bit. Okay, I just did a search and it's called a Scat Mat. Go to this site to read about it:
http://dogtrainingstore.com/Scat_Mats.htm
It seems like a great way to keep pets off furniture, so you might want to give it a try.