There are a lot of great posts here, and I echo their advice: Staying thin and getting MODERATE amounts of exercise is the best advice (HD or not!). Swimming or water therapy works wonders, and they tend to love it.
Giving joint supplements should help a great deal. Look into Cosiquin--we use it on our Newfs, and it has made a big difference, especially with Nell, who is approaching 11 and has very bad hips.
I know it sounds very scary, but I've had Newfs with HD live to be 13. We're very lucky that medicines and foods are so much better than they used to be.
I would also encourage you to keep looking for a vet you like and can trust. We have always had 2 vets for our Newfs, one local for the "basics" like shots, emergencies, etc., but we also have our "preferred" vet for things like consultations about HD, arthritis, etc.
There is no better feeling than having a vet you feel good about, and nothing worse than not trusting someone with your fur-kids, so even if it means a longer drive, it can be well worth it.
Good luck, and keep us all posted!
Pat
Nell--almost 11
Annie--still a puppy at 8!
Molly, Sam, Katie: at the bridge, but always in my heart