Shock cooling is a rapid, uneven cooling of the cylinders that can lead to cracking. Here's what I do to prevent it. Before I reduce power for the descent, I increase mixture about a minute prior, which reduces cylinder head temps. After that, I slowly reduce power to about 1700 rpm and decrease airspeed, which limits the amount of air passing the cylinders, again reducing the chance of shock cooling. I keep power on at this level all the way down to my desired altitude. The trick is not to pull the throttle from the firewall to idle and dive for the ground. Tough on the cylinders.

As part of my training I practice engine failures, but I do those at 1600 RPM. I've found if I put in 20 degrees of flaps at 1600 RPM I can simulate the sink rate/forward speed of a power off/best glide speed descent. Works just as effectively for training, I get to determine if the field I picked was within gliding distance, etc without risking cylinder damage. May not be EXACTLY as realistic, but when you own the plane, you want to minimize damage, because neither cylinders nor A&P mechanics are cheap.