Well... I run a savage.
After every run in dirty conditions, I hit it with a garden hose and use canned air to blow standing water off the carb needles and servo's after the days beating. Drop some ARO down the gp hole.
Before I go out the next day, I give everything a once over to make sure no screws worked loose of fell off. Also check the air filter to see if it's dirty.
Every gallon:
1) Clean the pullstart oneway with alcohol
2) Pull the cb and rough up the shoes with fine sand paper and sand the inside of the bell.
3) Check the bell bearings.
4) Check the header gasket to make sure it's still gasketing.
5) Inspect all cvd's and outdrive cups for wear.
Short of that, I listen to it when I drive for any gear skipping or anything. Anytime something doesn't sound right, I shut it down and find the new noise.
About every 10 gallons, replace engine.