Hi Fiona,
My mom absolutely did not want her schnauzers up on the furniture either. She did the "Coin Can" method ;) It took about a week for it to completely work. She put coins in coke cans and placed them on strategic places upon the furniture. "Strategic" being the soft places that if the dog jumps up, it would fall nearby/on the dog, but remain on the furniture. That was if the dog jumped up again, they'd move again ;)
She'd reinforce the jump-off, by praising her dogs that they stayed on the floor or went to their bed. She used the coin/can method all of the time.... They live on a split level...dogs aren't allowed on the staircase when the doorbell rings... barky too much, etc... Works for her.
Another thought is what people with cats use. Line along the counters and furniture with aluminum foil, shiny side up. I don't know it will work for a small dog, but it might be worth a try ;)
toodles