When it happens, I just talk to the neighbours. "Excuse me, but it looks like you've accidentally put your lawn chairs / setup your awning / parked your car on my site. Would you mind moving them over to your site?" Just be polite and assume that it was an honest mistake. No need to make it a big confrontation; just keep things pleasant.
On a rare occasion I'll get some puffed-up redneck or ornery old coot who takes offence. In which case, I'll just apologize for bothering them and call the park office, explain the situation and ask for someone to come down and speak to the neighbours.