Opening the oven door too often
Once the turkey is in the oven, resist the temptation to open the oven door to see how it’s doing until you start checking the bird with a meat thermometer, roughly half an hour before you think it will be done.
Basting the turkey
What about opening the door to baste the turkey, though? Most authorities today say that basting makes little difference in either the appearance or the moistness of the finished bird. It also prolongs the cooking time both because the drippings cool the meat’s surface slightly and because frequent opening of the oven door lowers the temperature of the oven interior. Besides, doing it is just one more thing to have to remember when you’re trying to coordinate an already complicated meal.
Not verifying your oven temperature
Make sure that the temperature inside your oven matches what’s set on the dial or display. Every oven is slightly different, and not all of them will have reached the temperature you want when the “preheated” signal goes off. You’ll need to leave a heatproof thermometer in the oven. Check it when you hear that signal, and if it doesn’t correspond to the temperature you’ve set, wait until it does.
Not using a meat thermometer
Unless you’re a professional turkey-roaster (does such a job description even exist?), don’t trust cooking charts or intuition. Use a meat thermometer to check when the meat is done. Supermarket turkeys often come with a small indicator inset into the breast that’s supposed to do the work of a thermometer, but by the time these pop up, the turkey is often already overcooked.
Inserting the meat thermometer in the wrong place
For your meat thermometer to work properly, it should be inserted in the right places. Put it into the thickest part of the breast on one side of the bird, holding it parallel to the bird’s neck and being careful not to let the tip touch bone. Then repeat the process on the other side. Next, insert the thermometer into the thickest part of the thigh, again avoiding the bone. Repeat that too.