After every ten flights or so, the carbon dust should be blown out of the motor with compressed air and the commutator checked to see if it is clean. If the motor is assembled with screws and the commutator is dirty, disassemble the motor and burnish the commutator with some extra fine steel wool or 1000-1200 grit sandpaper.
Some helicopters (ECO, etc) have open-faced bearings. These bearings are not sealed, and because the ball-bearings are exposed, they can become contaminated with dirt and grit.
If you notice the main rotor shaft bearings grinding or not rotating smoothly, then they should be removed and cleaned in some good solvent or cleaned with compressed air. If they still sound or feel funny, they should be replaced.
The tail rotor shaft bearings may incur damage if the tail blades touch the ground, especially on the ECO 8/16. They should also be checked periodically and cleaned or replaced if necessary.
Tail belts tend to loosen over time especially if the tail boom mount is loose. They should be checked and retensioned periodically.
O-rings will last longer if they are unhooked from the battery mounts when not in use. If they are left under tension continously, they will develop cracks over time. The o-rings should be checked periodically and replaced if necessary.
If the main rotor blades appear damaged do NOT fly with them. They can explode in flight which is very dangerous. You should replace any blades which appear damaged.
Ball links (especially above the swashplate) will wear out and require replacement as they become loose.
Servos may start to jitter as they wear. This may be caused by two problems:
Worn potentiometer. This requires the servo to be replaced or sent to the manufacturer for repair.
Chipped gear teeth. This can be fixed by replacing the servo gear set.
Ball bearings should be checked periodically for wear.
Insert a shaft through the ball bearing and push on the shaft at right angles to the bearing while rotating the shaft. If the bearing does not rotate smoothly (feels "notchy"), then it should be replaced.
If you have a boom strike, the tail boom can be straightened by pushing a 3/8" dowel through the tail boom.
If you let the Corona come down too fast it may "bounce" off the ground and break a main gear tooth and/or trash a bearing. So, the main gear should be checked periodically for missing teeth and the main rotor shaft bearings should be checked for smoothness.
Be sure to check HS-81s for broken gear teeth after each hard crash. They may rotate properly through the whole rotation, but they may have a broken tooth somewhere.
I consider the HS-81 gears a "sacrificial" part...they break to prevent other things from being broken, and at $3-$5 a set they are fairly cheap.
There isn't much maintenance required on a Piccolo. The stock brushed motor (the 280 or 310) should last 300 flights or more without maintenance if properly broken-in.
If the antirotation pin breaks off the swashplate but the stub is left, then it can be fixed with a small nylon tiewrap. Cut the tip from the tie-wrap and lash it the remnants of the antirotation pin by using several loops of cotton thread. After tying, put a drop of thin CA on the thread to harden it.
The one-way bearing in both plastic and aluminum autorotation hubs has been known to become loose. Usually when this happens, the one-way bearing can be pushed out of the hub with firm finger pressure.
This can be easily fixed by roughing up the bearing and autorotation hub mating surfaces with 200 grit sandpaper and using thick CA to reassemble the unit.
The tail blade grips (67542) should be checked periodically to make sure they are not too loose. If they are, be sure to unscrew them from the tail rotor hub and apply fresh loctite to the screw before reassembling.
The main rotor shaft bolt (67599) is rather soft and can become bent in a hard crash. Be sure to check this bolt if the main rotor blades have hit anything. If the bolt is bent, be sure to replace it and do not fly with it.
The battery holder O-rings (67587) will need to be replaced about once a year because they will start to crack. These can be replaced with the stock Ikarus parts or you may be able to find the O-rings(30mm ID x 3mm thick) cheaper elsewhere.