The key to breaking in any engine is to not allow the engine to remain at a set rpm and load for any length of time. Vary the load and rpm with throttle and/or mixture, but try to not allow the mixture to lean out to peak rpm setting and try to keep the rpm around 3/4 of max at the beginning. Here's how I break in a new engine:

I usually use a prop with an inch or two more pitch than recomended for the engine. This keeps the rpm down a bit. I start the engine and tune it to the rich side to the point it will transistion, but not smoothly. This is usually just a bit lean of 4-stroking. Run the first 15 minutes alternating between full and half throttle every 15-20 seconds. Allow the engine to cool completely.

Second 15 minutes of run time I lean the mixture to about 1000 rpm shy of peak and alternate between full and half throttle every 15-20 seconds. Allow the engine to cool completely.

Third 15 minutes I tune the mixture about 1000 rpm below peak, leave the throttle full open, and turn the needle every 15 seconds to vary mixture between 1000 and 5000 rpm below peak. Allow the engine to cool completely.

Fourth 15 minutes I tune the mixture as if I was going to fly it, about 200 rpm below peak, leave the throttle full open, and turn the needle every 15 seconds to vary mixture between 200 and 2000 rpm below peak. Allow the engine to cool completely.

Fifth 15 minutes I install the prop I plan to use and tune the mixture as if I was going to fly it, about 200 rpm below peak, leave the throttle full open, and turn the needle every 15 seconds to vary mixture between 200 and 2000 rpm below peak. Allow the engine to cool completely.

First flight I tune a bit rich, but with good transition. I avoid full throttle and vary the throttle setting every 30 seconds or so. I gradually add more throttle and more time at each setting for 3-4 flights. Finally, about the 5th flight, I lean the needle for maximum in-flight rpm and adjust low speed for idle and transition.