OK, so first, save the beautiful juices that you have from the turkey that is inside the bag Jenny told you to use. I usually rub my turkey with butter before putting in, mainly because it makes the gravy taste heavenly and I am all for calories on Thanksgiving. So, you have this pot full of juices (at least the bottom should be covered about an inch or so deep, hopefully you have a bit more than that.)

In a separate bowl mix 1 Tbsp corn starch, 2 Tbsp. flour (white does work best, sigh) salt and pepper to taste and enough milk to make a paste. Then add more milk til the paste is runny. Add it to the simmering juices, and whisk the heck out of it! Keep stirring over low heat til it thickens. I usually add a little more milk as it is thickening to make sure that I have enough gravy (or water, if you'd rather not use that much milk). Anyhow, if it won't thicken after a few minutes of this, take some out, mix it with more flour (another Tbsp or so) til you have a paste again, mix the paste back in, and whisk again.

Works every time--for me anyway.