Hold the glass at 45 degrees pour it up until the head reached the little 'Harp' logo on the side of the glass, or about two thirds of the way up for a non Guinness branded glass. You then let it settle for at least two minutes. Then top it up, so you have an nice dome of head on the top of the pint. One extra tip is, for the last third 'topping up' phase, to push the tap away from you, rather than pulling it towards you. That'll put less nitrogen into the pint, and therefore make sure you get more beer, and less head.