As PD affects small muscles first, the minor wriggles and adjustments people normally make get diminished, this is one of the reasons turning in bed can become difficult, or even sitting in a car - making you feel as though you are leaning back too much. Sliding and leaning when sitting are part of the same problem, so exercising core muscles, and generally doing some exercise that works the little muscles will help. Tai Chi, yoga, swimming, walking are good, and I can recommend the John Argue book of exercises too.

I am useless these days at keeping up exercise so I am passing on what I know works for me even though I don't work at it