What I was taught was to get the engine used to running in all RPM ranges, from slow idle to WOT for breaking in. Also use a rich oil to gas mixture for the first tank.
try to run the engine steady in increments of 5-10 mph while also running through the entire powerband from idle to WOT every few miles. ride 20 mph for a few miles, then 40 mph for a few miles. then open it all up and let it off all the way back to about idle. Then maybe 30 mph, then 50 mph, then 10 mph and so on. then back off and/or open up again.
Anybody have any better advice? I have never had problems doing this on rebuilds or new engines.