I love moist turkey and this is the moistest I've ever tasted! I've been using this method for years now, but it takes quite a bit longer to cook at this low temperature, so give yourself plenty of planning time.
I soak all my meats in salt water first to remove the blood. 30 minutes up to several hours is fine. If you can't find a container big enough to soak your whole turkey, you might consider using a cooler, and then washing it well. It also greatly enhances the flavor.
Time: ~1 hour per pound, plus one more hour (your bird's weight + 1 hour, less for small birds)
Rinse turkey thoroughly, including cavities, and pat dry with paper towels.
Stuffing is optional, but internal temperature of stuffing should remain at 165 deg. if stuffed.
Rub outside of turkey with olive oil and any other seasonings desired.
Place breast side down on rack in roasting pan, with thermometer in meatiest part.
Place in preheated 300 degree oven and roast for one hour (this kills any bacteria).
Reduce temperature to 180 or your oven's lowest setting above that. Roast 45-60 minutes per pound (3 times longer than standard). The larger birds require 60 minutes per pound. It doesn't hurt anything to cook it for several hours longer after it's done, in case your timing's off.
Roast until the meat thermometer reaches 180 deg.
No basting required! Lots of juices for gravy. Be prepared that it will be so tender that the bones may fall off when you cut it!
Warning: Some digitally controlled ovens have an automatic cut off time. You may want to consult your owner's manual about this, or set the alarm to check on it in the middle of the night! If your oven just uses a dial, you're good!