I don't buy yogurt outside. I make small batches at home. Here's my method:
* Heat 1/2 L of milk, just to boiling point.
* Set it on the counter to cool. It's cool enough when you can stick your finger in and hold it for a few seconds (not too hot to kill yogurt bacteria)
* Then you can add a spoon of yogurt from previous batch (I've found less yogurt makes it set better...add too much and it comes out chunky)
* Stir it well, then cover and let it sit.
In terms of getting the right temp for incubation.....the heating pad is a great idea! I also:
* Keep it near the stove while i cook dinner. It can set in 3 hours this way!
* Put it in the oven, with just the light on, so it gets warm, but not hot.
* Don't leave it overnight, if your kitchen gets cold.
I've never heard of needing to sterilize jars for yogurt. The whole thing about it is that there are bacteria that keep the milk eatable even if it sits out, right?