A heads-up to this effect should be part of the preflight briefing, especially for first-time passengers. They'll intuitively know that you'll need to concentrate on 'playing pilot' during the takeoff and climb to cruise. But after hours of enjoying your sharp wit and insightful commentary during the cruise phase, it won't be entirely natural for them to realize that you need to focus on flying the airplane. It helps a lot if you told them before engine start that, at some point toward the end of the flight, you'll have to ask them to clam up. If you have an intercom that can exclude the pilot, they are welcome to continue talking among themselves. Actually, this is a good idea, since there's a greater likelihood of passengers getting nervous if they all of a sudden have to stop talking and 'realize' they're in a small airplane about to land. One solution is to assign tasks to passengers, such as watching for other aircraft, looking for the runway lights (for front seaters), and so on.