I just tried this recipe the other day. I got it from http://www.nourishingdays.com/
Plain Yogurt in the crock pot
Turn your crock pot to low and pour in 1/2 gallon of milk.
Heat on low for 2 hours and 30 minutes. (if you want a thicker texture do it for 15 more minutes)
Once 2 hours and 30 minutes have elapsed turn your crock pot off and unplug it. Let the milk cool in the crock with the lid on for 3 hours.
After 3 hours remove 1-2 cups of the warmed milk and place in a bowl. To that add 1/2 cup of yogurt with live active cultures and mix very well.
Pour the yogurt-milk mixture back into the milk and whisk thoroughly.
Place the cover back on the crock and wrap the entire crock pot in a thick bath towel or two.
Let it culture overnight, 8-12 hours.
In the morning stir yogurt (if desired) and store in glass quart jars or a container of your choice.
For optimum texture, refrigerate for at least 8 hours before using.