The best mattresses will be firm enough to support your body weight, but not hard enough to put pressure on joints like the hips, knees or back. So it becomes a matter of finding the right balance between these two extremes that works for you. One basic rule when searching for a new mattress is to choose one that is slightly firmer then you would normally need. You don't want to go to any extremes here; remember, it should be only "slightly" firmer than you normally desire.