Incubate the yogurt at 110-115 degrees F for the next 6-8 hours, up to 12 hours for a tarter flavor. I've found that my heating pad inside a cooler maintains this temp perfectly. However, most heating pads will have an automatic shut-off, so you'll have to babysit it a bit. Pack any dead air space in your cooler with wadded up towels for better insulation. My long probe thermometer is resting right against the side of one of the jars.
You could also try setting them in the oven, if you can set the heat that low. If you're just itching to spend lots of money, you could buy a commercial yogurt maker, which I believe maintains a constant temp as your jars incubate in a water bath. I prefer my free setup, using common household items.
That's it! Refrigerate the yogurt and it should keep for at least a month or two. You'll notice that the curd separates out from the whey after it sits awhile, so give it a stir and you're good to go. This makes a great base yogurt to blend with fruit, granola, etc. It's also good over cereal in the morning, or whipped up as kind of a yogurt drink.
Finally, you can pour the finished yogurt into a strainer lined with cheesecloth and let it hang for several hours to make a "yogurt cheese" that is much like cream cheese. You can make the plain version of this yogurt and then add any sort of flavorings to the finished cheese for a fancy spread.
Let me know what you think!