I've experimented a lot. Here is my current favorite method:
Bring 1 quart of whole milk to a boil.
Put in a cool water bath until the milk is 115 degrees F.
Meanwhile, boil a teakettle of water and pour the boiling water into the glass/stainless steel container that you are using (this sterilizes, but also warms the container).
Put 4 Tbl. of yogurt in a bowl.
Dump out the boiling water and dry container with a clean towel.
Preheat your oven to 200 degrees for 2 minutes, then turn the oven off.
When the milk is cool enough mix 1/2 cup of warm milk with the yogurt and whisk. Whisk all of the milk and the yogurt in your clean container.
Cover and sit in the warm oven for 5 hours.
I used to make it overnight, but 5 hours is the perfect time- it doesn't get very sour. You can also add some dry milk to the milk/yogurt mixture, which makes it thicker.
Good luck, let me know if you have any questions.
By the way,
Some yogurt makes a better starter than others. I buy Butterworks Farm yogurt when I don't have time to make it cause its yummy- but it makes a lousy starter. Stoneyfield whole milk yogurt makes the best starter...