A service is usually just an oil change, new plug, clean the carby and so on...
not so simple, here is how I service my machines,
1. Change gearbox oil every 5 hours.
2. remove and clean your airfilter every 5 hours depending on your riding conditions (dusty conditions, clean it more often), re-oil it with quality airfilter oil before installing. buy a new filter once a year.
3. no need to replace the sparkplug if it still fires, keep a few spare ones with you when you go riding along with the appropriate tools and replace it if the plug dies.
4. take apart and clean\grease your linkage\swingarm\shock and you steering-stem at LEAST once a year, more often if you ride alot and even more often if you pressure-wash the bike after riding. replace bearings and sleeves if they are rusty or bad.
5. Replace the piston about every 30. hours of riding, more often if you abuse the engine (burnouts, stressing the engine without letting it warm up, keeping max RPM for extended periods). inspect the cylinder for wear, scars or marks.
6. the rest is actually pretty logic, repack your silencer when the bike gets loud, replace your pads if they are thin, replace your clutch if it starts to slip, replace your sprockets and chain BEFORE it snaps and so on...