You should try to maintain a position which is in a direct line to where the cutter is trying to cut. Turning to look away from your man gives them the opportunity to get away from you. It is also a good idea to focus on the belly-button of the cutter, as this is the centre of mass, so that head fakes and leg fakes can be ignored. You should go for the lay-out block when the disc is thrown. The problem with face guarding is that a disc can often come close enough to get a block on, but because you are looking away you miss the chance.
Most defenders prefer to watch the disc as well as their man.
This has the advantage that you are more likely to be able to get a block on the disc since you can see when it is thrown, but the problem is that you are not focussing all of your attention on the person you are marking. This tends to make it easier for them to get away. There are different levels of this sort of defence, ranging from the occasional glance toward the disc, to facing the disc and trying to defend by looking over your shoulders. The most common is side marking, where the defender stands slightly on the open side of their man, with the thrower is almost directly to one side and their man directly on the other side. By turning your head slightly you can see both the disc and your man.