The problem with roasting a chicken too much to make the skin crispy is that it can make the meat as dry as cardboard. What you can do is to brine the chicken before roasting. This will add moisture to the meat, as well as flavor. For a medium-sized bird, you can start roasting at high heat, about 400. This should give you crispy skin with tender enough meat. Another option (high fat, though) is to rub butter or some similar ingredient under the skin before you roast the chicken.