Parkinson's causes the GI tract to dry out from one end to the other. So I get dry mouth, pills are prone to getting stuck in my esophagus halfway down, and constipation. Stool softener (docusate) works great for the constipation. For the dry mouth I have a whole array of mouth moisteners and hard candies. Even so I sometimes end up with a sore tongue from lack of saliva.
The official use of pilocarpine is to trigger the production of saliva in cancer patients, however a doctor can prescribe it for PD if s/he sees fit. It works, but there are downsides: it is short acting and only lasts for a few hours. Also, if I take a bit too much I end up with a hot flash - a feeling of heat coupled with a cold sweat, particularly if taken before bed.. My sensitivity varies - sometimes the 5mg dose is right, sometimes I need to split the tiny 5 mg pill with a pill splitter. It's not perfect, but I do find it is helpful, and thought it was worth sharing.
Update: since posting this I have switched to Cevimeline which costs more but is less prone to the cold sweat side effect.