For elderly dogs, and especially those with arthritis, I would also highly suggest a good joint supplement. Please keep in mind that not all supplements are going to work for all dogs, so you might be trying out a few of them in your search.
Because all three of my dogs have varying conditions that require it, I've got them on Platinum Performance complete joint
, which goes in their supper in the evenings. This is a top of the line supplement and is priced accordingly, but for us has been worth every penny. They are extremely quick to ship too, so you can get this quickly. I also have them on a morning supplement from "Zesty Paws
", also an excellent supplement, price per pill (chewables too).
If you don't want to wait (and I urge you not to), you can pick up some glucosamine/chondroitin tablets at Walmart and give them to her, as the human formulation is no different than that for dogs. Get the TRIPLE strength. Just pop them in some cream cheese or something, and let her have them. It'll take a week or so for any noticeable improvement, but these truly will help.
I've also got my youngest on a supplement called microlactin, as she's got a luxating patella, and it has helped her tremendously. This and other things are discussed in a thread I started here
. Also suggested for joints is Cetyl-M (the Platinum Performance supplement contains this, but in small a small amount), which is next on the docket if the microlactin doesn't work in the long term.
You cannot go wrong with a good joint supplement(s) in an middle aged/elderly dog, and in a great deal of cases, they spring back miraculously. What's more, is with water-based supplements like these, you can give them in quantity until they work, as it's virtually impossible to overdose; unless of course you're supplying bucket sized or other absurd quantities.