My old boy, a GSD x, became rather stiff in his latter years. The vet diagnosed arthritis and prescribed two things. Glucosamine daily - she told me to buy the human stuff, as the vet-authorised version was very pricey - and Rimadyl to use when I knew he had a big day coming up, for a day or two before, the day itself and a day or two after, and any other time if he seemed particularly sore.
And gentle, regular exercise; several short gentle walks a day, not the big all-day hikes. And keep the amount of running and jumping to a minimum; walking and trotting better than flat out. Definitely make sure he's had 10-15 minutes warming up before doing anything more strenuous.
We still did the big all-day hikes for a good few more years, but I bracketed them with Rimadyl, and I helped him over stiles. And he accompanied me on my thrice-weekly 3-mile jogs for a few more years also, but towards the end they were too much for him.
I also started to give him a hot water bottle on cold nights, and a second duvet to cover him (he slept on a folded duvet.) And made sure he got dried off and warmed up quickly after getting wet.