To prevent a turkey from becoming too dry as you cook it, start with a fresh bird that has never been frozen. Freezing damages the cells in turkey meat, allowing each cell to lose more moisture when defrosted. Also, dry or wet-brine the bird before roasting it. Both dry and wet brines help the turkey retain moisture while cooking, but dry brines are easier to use on very large birds.
Instead of roasting a large turkey, consider cooking two smaller birds to ensure the turkeys cook evenly without drying the white meat. Also, roast the turkeys on a rack upside-down for the first hour of cooking time. This technique allows the juices from the bird to self-baste the white meat. Rotate the bird so the breast side faces up after an hour.
Always use a meat thermometer to prevent overcooking the turkey. Test for doneness by inserting the thermometer into the turkey thigh, making sure to avoid the bone. The thermometer should read 170 degrees Fahrenheit. Allow the turkey to rest for 20 minutes outside the oven before carving to allow the moisture to distribute itself throughout the bird.
In addition to brining, rub softened butter underneath the skin of the turkey to help preserve the moisture content of the bird. Also, avoid trussing the turkey tightly, which increases the cooking time.