The first thing I do is try to correct any postural problems. Usually with older clients their problem has become structural so a lot of stretching is needed to try and regain normal ROM.
I then try to strengthen the muscles in the right area. A lot of this comes down to finding ways to train around pain. If the client is experiencing pain with every rep, i terminate the exercise. If they only experience pain when they let their posture drop etc.. then I give them prompts to keep good posture and we keep going. For instance, forward ab flexion seems to irritate clients who have bad backs/necks. Rotational movements however seem to be fine.
The important thing, is do not do anything that risks them getting worse. Even then we are not physio's, with injured clients we have to do exactly that. Make the problem worse and you'll have someone out there enthusiastically telling everyone they know how bad you are at the job.
Also, don't be afraid to refer clients on. If you don't have the skills or knowledge, send them to someone who does.