Our little Ethel lost mobility in her back legs nearly two and half years ago. One piece of useful advice that we received from our friends at Integrative Pet Care is to encourage her to sit in proper, functional formation as much as possible.
When left to their own devices, Ethel's legs stick straight out and toward her head. Not only does she look silly that way, but it lessens the chances that her feet and central nervous system will share any kind of useful feedback with each other. By coaxing her legs into a good square sit, we are promoting whatever functionality her legs still have.
I believe this is one of the most important things you can do for a dog with decreased mobility in their hind limbs. Keep those nerve pathways open and functioning!