One of my friends raced MX on dirt bikes, and the guy that told him how to break in a new top end on a 2-stroke was let it warm up completely, ride it easy. Let it cool down completely, Warm up again Ride it a little harder, let it cool down and then have at it. He said the cooling up and down will let the rings seat completely.

Not sure if this is true, i did it with an LT250R and it worked pretty good, i never had a lick of trouble with it.