Make yogurt at home

I like yogurt a lot, especially in my morning smoothie. I figured out how to make it from scratch, thereby giving me total control over the ingredients, and also saving me money.

1. I started by sterilizing everything I was working with. I accomplished this by taking a big pot, and boiling my utensils and the jar in it. I don't know how necessary this was--I presume the yogurt culture would out-compete any bad microbes, but being careful isn't a bad idea.
2. To make the yogurt, you need 1/2 cup of starter. I used plain, unsweetened yogurt from the store to make the first batch--but be sure to read the label, make sure it lists live, active cultures (the more varieties the better). Subsequent batches can be made with a 1/2 cup of the finished homemade yogurt as your starter.
3. Add the yogurt to a quart of milk of your choice. I used plain soy milk, which though plain, does contain added sugar. This is good because sugar takes the place of the lactose in dairy milk, giving the bacteria something to feed on and convert to lactic acid--which gives yogurt its tang. (Adding even more sugar might result in more lactic acid and therefore more tang--I plan to experiment.)
4. Keep this mixture at about 110-115 F. degrees (any hotter and you'll start killing the culturing organisms; colder, and you'll inhibit their growth) for about four hours. I mixed it in a one-quart mason jar, which I then set in a pot of warm water, and monitored the temperature with a candy thermometer. Whenever it cooled a few degrees, I removed some of the water, and replaced it with hotter water. You could probably set the pot on a heating pad and have an easier time of it. Or buy yourself a yogurt maker--but I'm all for doing it old-school.
5. After 4 hours fermenting at 110 F. degrees, it will look, smell and taste like yogurt. Longer fermenting makes more tart yogurt. Use it right away, or for better flavor, put the yogurt in the fridge for about 12 hours.
6. When you start running low, save a 1/2 cup of your homemade yogurt to use as a starter, and repeat the process.

Notes about soy: I'm trying to limit my dairy milk consumption, and my corner store sells decent soy milk for $1.50/quart. Soy milk yogurt has a thinner consistency than dairy, and is pourable. You can thicken it with agar, guar, whatever, but since I use it in smoothies, I don't bother. The flavor of soy milk yogurt is almost identical to dairy milk yogurt, in my opinion.