My DS went through a phase of favoring the right breast. I just always offered the left breast first and attempted a couple times to get him to feed from that side before moving him to the right side. My supply on the left did seem to drop a bit, but it was a short phase (A couple weeks.) and then he stopped showing preference from side to side so he was able to bring the supply in my left breast right back up. In my experience, it was just a phase and he eventually grew out of it on his own.