I submerge my soon-to-be-yoghurt jars in 42°C hot water in a pot, then put that next to the radiator during the winter months, or in the sun during the summer months. Check up on the temperature maybe once or twice over the course of 6-8, then put in the fridge overnight.

I usually make 1.5kg of yoghurt that way, and it keeps for about one to two weeks — but it never survives that long (2-person household), so I make yoghurt around every three days. We use it for breakfast cereals, yoghurt with jam (home made jam for added benefit :-) and ayran. It's fantastic!

For safety, you should keep yogurt warm as it can be out-competed by harmful things, I once tried using something like this: http://smile.amazon.com/TAYAMA-YX-45A-Tayama-Vacuum-Pot/dp/B...

But I found it difficult to get a thermal pot down to the right temperature range to begin yogurt and switched to making only kefir. Kefir is less likely to lose to a contaminant at room temperature as it is a diverse cocktail.

I make yogurt by heating the milk to 190F, holding it there for a minute or two, then taking it off the heat. The 190F helps break down the proteins (casein if I remember correctly) to help the yogurt setup. If your milk hasn't been pasteurized, this also pasteurizes.

I let the milk cool naturally to 115F, then I mix a tablespoonful of the warm milk with two tablespoons of my starter yogurt (for a gallon of milk). I continue to mix a tablespoon of milk into the starter until the starter is pourable. It then gets added to the pot of milk and stirred. If I'm making skyr, the rennet goes in at this stage.

The pot is usually at 112F-114F and get wrapped in towels and thrown into a warm oven, I usually heat the oven to it's lowest setting (200F) while the milk is warming, then turn the oven off while the milk is cooling. The milk sits in the oven overnight. The 105F-115F range is the "ideal" temperature for the bacteria.

In the morning, the curds get sliced, strained, and jarred.

Once you get used to it, making yogurt once a week takes an active 30 minutes of my time.

If you use a mesophilic (rather than thermophilic) strain you don't have to keep it warm: it ferments at room temperature. The most popular strains in the US are all thermophilic, but Finnish viili and a bunch of other (primarily Scandinavian) strains are mesophilic and you can just plop it on your counter overnight. "Bad" bacteria won't have a chance as long as you started with clean materials.