Everyone has thier own method I have heard and seen em all. There are alot of variables to this question. I would say just ask your motor tunner or one you trust. Even with them you get alot of different answers. The most important thing is that you do it. If you dont you hurt performance and wear out your motors alot faster. Pick a method and go with it and refine it to your needs.
Heres what I do. I run it at 2-3 v for 2 minutes at a time till the brush face looks right. then cut the comm and break it in for 30 seconds more. At least I try to do it like that every time. I used to never use a fan but lately I have been it takes a little longer to break in that way I think because of heat not being as high.
Here is the place this question should be asked at.
Big Jims Forum