In the crates that have one, it's a good idea to keep an eye on the temp gauge. Around 80 degrees C is a good temp to shoot for. Running too cool will decrease your power. I've noticed the SE5a will run cool if the radiators are full open, so I close them up to about 60%. As a general rule, when you gain altitude, they need to be closed as the air gets colder and thinner.
The rotaries don't have radiators, so there are no shutters to control. Everything has to be managed by adjusting the mixture and throttle settings. Luckily, they don't require much fiddling to keep running steady. With all of the aircraft, rotaries and inlines, it's not a good idea to run them wide open all the time. Use full power for takeoff, and then decrease to 80 or 90% throttle.
Throttling back or using the blip switch in a dive is also important. Diving with the engine throttled up will cause you to exceed the maximum RPM for the motor. Do this for too long, and things are going to seize up or throw oil.
On the crates that have an inline motor and no temp gauge, look for the radiator cap and make a mental note to glance at it now and then. If it's steaming, and your plane is leaving a white trail, open the radiator some more. The Albatross DII has no radiator shutters, so if you're flying that one, just throttle back if things get hot. Engine meltdown won't happen instantly, but one must still keep an eye on things.
There are still some planes I haven't even flown yet, so that's all I can say with limited experience.