How would you brief passengers who will try flying your plane?
I find that virtually all the passengers I've let fly for the first time were already so scared of the controls that no "safety briefing" of that sort was necessary.


  Don't fight me on the controls.
  
  If I say let go, let go immediately.


If you suspect there's even a remote chance of either of these problems, maybe you shouldn't be letting this person fly your plane.


  Don't move any of the controls very rapidly.
  
  Don't move any of the controls to their limits.


In most small airplanes without hydraulics, it takes quite a bit of force to deflect any of the controls rapidly to their limits. And as soon as your first-time passenger starts to push on the controls, they're going to feel increasingly strong g-forces as the plane responds. For first-timers, this is usually a pretty intimidating feel. If someone is pushing too hard, they're going to feel it, and almost certainly back off or let go entirely.



"Positive transfer of control" using 3-step acknowledgment such as "You have the controls" / "I have the controls" / "You have the controls" is a good idea for flight training, or for flights with 2 pilots taking turns at the controls; but in my opinion it's not necessary for a joyride with a friend. As the PIC, and the only licensed pilot in the cockpit, you need to be monitoring the situation at all times anyway. If you decide the situation requires you to reassert control, you can simply ask them to release the controls. If you can't trust the person to do this promptly, you probably shouldn't be letting them fly your plane anyway; a preflight safety briefing isn't going to solve that.



To be as safe as possible, you can take the following steps before allowing your passenger to fly (some of these you already stated):


Select a location away from controlled airspace, busy airway/routes, or other special-use areas like drop zones.
(Optionally, if you don't mind the extra radio chatter) Tune a nearby ATC radar facility (approach/center) and request radar advisories. Tell them you'll be maneuvering in a certain area.
Climb to a sufficient altitude to allow good clearance from all terrain and obstacles (antennas, etc.), and to allow enough altitude for recovery if your passenger should somehow manage to stall or enter a steep dive or something.
Trim for straight &amp; level flight at $V_A$ for your current weight (lower than the gross-weight $V_A$ you get out of the book).
Make a set of clearing turns and scan for any traffic in the area.
Give your systems a once-over and verify you have sufficient fuel.
Hand over the controls!




You didn't specifically ask about this, but I've flown with a lot of first-time passengers, people who have never been in an airplane. Both children and adults. Many of them are understandably quite nervous. Here are some things I try to do to make it as easy, fun, relaxed and stress-free as possible for them:


Give them a simplified, plain English description of the flight. Tell them exactly what you plan to do and what order things will happen in. E.g.



  First we're going to start the engine. I'll get on the radio and get clearance from the control tower. Then we'll taxi out to the runway and stop for a few minutes while I go through some checks to make sure the plane's in good shape. Then we'll take off and head out to the east to do some sightseeing. While we're up there, if you're comfortable and would like to try your hand at the controls, we can do that. When we're done with that we'll turn around and head back to the airport, land, and taxi back in.



Don't use fancy words or terms that a non-pilot wouldn't understand.
Help them enter the airplane. Show them how to open and shut the door. Show them how to buckle and unbuckle their seat belt.
Ask them if they have any questions.
Do your normal preflight safety briefing, again using simplified English and avoiding pilot terms. Limit it to points that are relevant to a passenger (avoiding the controls, staying quiet if asked, opening the door). They do NOT need to hear about what you're going to do if you have an engine failure on climbout. If this is a normal part of your preflight / before takeoff briefing (as it should be), say it to yourself mentally, not out loud.


During the flight:


Talk your way through everything you're doing. When you're ready to do your run-up, tell them:



  I'm about to run the engine up to power to make sure everything's running smoothly. I'm holding the brakes so we're not going to go anywhere, but the engine will get pretty loud for a minute.



When you're doing your magneto / prop governor checks:



  Now I'm going to do some things to the engine to make sure it's running right. It's going to make some pretty funny noises, but it's perfectly normal.



Talking through everything accomplishes a few goals:


Avoids startling the passenger with something unexpected. A prop governor check sounds really bad to somebody who knows about engines but nothing about planes. A stall horn in the flare may be perfectly normal to you but a loud buzzing alarm is the last thing the first-timer will be expecting to hear. Tell them about it ahead of time.
Makes the passenger feel like they're a part of the flight and not just dead weight in the plane.
Keeps you focused on your tasks as PIC and discourages you from getting distracted in conversation with your passenger.

Make the flight as smooth as possible. Don't overcontrol the airplane. If you drift 100' from your intended altitude, don't pull 2 g's to get back up to it. Make it slow and gentle. They're not a pilot, chances are they're not going to know you drifted 100 feet. They will know if you jerk the thing around. DO NOT hot dog the airplane. You may be bored with slow, gentle flight, but your passenger isn't.
Speak slowly, clearly and confidently.
Smile and try to look like you're having fun. Your first-time passenger is probably feeling pretty vulnerable. When people are vulnerable they feed on the emotions of the people around them. If you look like you're having the time of your life, they're probably going to have fun too. If you look angry, ticked off or scared, you're going to scare them too even if the flight is perfectly smooth. This is especially true and critically important with children!


If/when they're ready to take the controls:


Show them how the yoke controls pitch and roll. Do some (very) gentle climbs and rolls to illustrate what various control inputs do.
Tell them to look outside and not to worry about all the fancy intimidating lights and dials in the cockpit. Tell them that the only tool a pilot needs to fly the plane in good weather is their eyes.
Tell them to relax and have fun! Assure them that they aren't going to crash the plane or break anything and that you're there watching and won't let them do anything unsafe.
After a few minutes, once they get the hang of basic control inputs, you can start to tell them about sight references: show them how to look at the horizon and tell if they're banked, climbing or descending. After 15-30 minutes you should be able to get virtually any passenger to be able to hold straight &amp; level attitude and make gentle turns. Being able to accomplish this is very rewarding for your passenger and makes them feel like they achieved something. It encourages their excitement about aviation and makes them want to fly again.
Don't forget to do your job: scan for traffic. Monitor your engine gauges. Listen for ATC calls. Don't stray into controlled airspace. "Maintain situational awareness."


I've had a great deal of success using these tips when flying with first-timers and children. In 16 years of flying I have had only one bad experience, with an adult passenger who had an unknown (to me) and pretty severe anxiety problem and had a panic attack shortly after takeoff. A couple of kids have fallen asleep on me (which I took as a compliment). Everyone else had the time of their life. And let me tell you... if you're someone who loves flying, there is no better feeling than sharing that with someone else and watching their excitement. It's the closest you'll ever come to reliving your own first flight, which hopefully you remember fondly because somebody cared as much as you do about being the best possible ambassador for aviation!