The throttle friction lock in my C182 works poorly in cruise, it slowly eases out to a lower power setting. What can I do to make it better? I've attempted to turn the lock nut harder but without result.
I suspect there is only one sensible answer to this, and that is to have a qualified mechanic check/replace as necessary.
To continue flying in the meanwhile I would add a check for throttle friction to my FREDA check which you probably already do every 6-10 minutes during cruise.
The throttle system is a metal plunger connected to a cable that pushes and pulls the throttle control on the carburetor. The friction device puts pressure on the metal arm of the plunger, usually using a set of pins which get pressed against the plunger as you tighten the nut. If the pins get worn down or the threads get loose you won't get sufficient friction to hold the plunger in place. I've never seen repair parts for these, you replace the entire throttle cable assembly.
You may be tempted to increase the friction available by wrapping the plunger arm with some sort of material. Do not do this, whatever you use could easily get caught and jam the throttle.