Engine Break-in

How do you break-in your new engines ? I usually just put them on my layout and run them, but I read on one of the forumns a hobby shop owner recomends that you run a new engine for about 45 min. varying the speed in forward and then the same in reverse. is this necessary ?, will it help engine performance and life, some of my engines are 15 years old and they still run good. I just bought 4 new engines and if this is a good thing to do I will do it before I start to run them on my layout.
Beeline

Some folks have shared some pretty comprehensive ways of breaking in their motive power. I usually just run a lokie on a static stand on my workbench at high and low speeds, both directions for an extended period of time. However, my best running Athearn, I broke in by pulling a 45 car train around a club layout for about 2 hours, without any ‘under hood tweaking.’ That lokie has lasted over 20 years with no change to motor or wheels, and it runs like a Kato, very quiet and smooth.