Beat the yolks over heat long enough to fill them with lots of air bubbles - but be careful to control the heat so the yolks don't harden. In order for the butter to enter, fill and expand the bubbles in the yolks, they have to be in a liquid state. Once you start adding the melted butter, don't apply heat again because that would cause the oil to expand too much and break the air bubbles, resulting in a separated sauce. For the same reason, don't reheat the sauce once it is completed. Don't let the sauce get too cool, either, because the oil will congeal and contract and cause the bubbles to burst. The ideal is to keep the sauce as close to body temperature as possible.