It could have something to do with genetics as well. I know even back before I started working out, before I knew what the hell a serratus was lol, mine were always very prominent. I have a wider build though too, so that might have something to do with it. Just some food for thought. Lying pullovers, and standing cable pulldowns always seemed to work well for me. And I rarely had any next day soreness of the serratus muscles.