Buttering under the skin is an excellent way of stopping the breast meat from drying out and at the same time adding flavour. I also smear the skin with olive oil or butter and sprinkle it with sea salt to produce a crisp golden appearance when cooked. Another way of keeping the bird moist is to lay a large sheet of buttered foil over it, but not wrapped tightly, or you simply produce a steamed, rather than roasted, bird.
Another way I sometimes use for large birds is to drape the bird with muslin dipped in plenty of melted butter which you remove for the last 30 minutes of the cooking for a brown finish.