Do not slice into your turkey to see if it's done.
This will dry our the meat and make it look ugly. There's also no reason you should be using the old "juices run clear" test to find out if your turkey is done.
BUY A THERMOMETER. They cost like $3.
Don't put it in the turkey the whole time the bird cooks. When it's time to get a read on your turkey (they cook at about 15 minutes/lb in a 325°F oven), stick the thermometer down into the thigh where it attached to the drumstick. Don't let it touch the bone. When the temperature of the thigh reaches 165°F, pull the turkey out, tent it with foil and let it sit for 30 minutes before carving.
How to make sure your thermometer is properly "calibrated": Fill a glass half with ice and half with water. Insert the thermometer and ensure that it reads 32°F. If it doesn't, leave the stem in the ice-water bath, then, using a small wrench, loosen the nut behind the dial and rotate the dial face until the need points at 32°F. Then re-tighten the nut.