Breaking in tires is essentially running them on the car for a little while so the pins wear the front edge off giving the pin more surface area against the track surface. This would only be done on medium to high bite surfaces. You could potentially chuck the tire up on a drill and use a sand paper etc, but i dont have any experience doing that myself. Somebody may have some tips in that reguard.
Slicks would be somewhat similar in that you want to remove the shine off the tire. You can run the tire or chuck it on a drill or dremel the surface with a drum.
Another need to break in tires is to work the foam inserts a little bit so they are a tad more forgiving. Others just cut the grooves or inside supports to facilitate that.