I had done it numerous times over the past couple of months and it turned out well. Then suddenly it hadn't--the day I gave the talk, I started a batch and it was runny. And the next day I got yogurt-scented milk! Holy moly I cannot even.
So I did some troubleshooting and experimenting and this is what I found:
You need to heat milk to 180 degrees to kill off any competing bacteria. I purchased a candy thermometer for $10 (it goes up to over 200 degrees) and took the temperature of the milk after the initial 2 1/2 hours of cooking. My slow cooker, set on low, only brings the milk to 160-165 degrees or so. And in order for the starter to incubate properly, the milk needs to cool down to 115 degrees. It's quite possible that the milk was too warm or too cool for the starter to work.
So what I did was this:
I got a candy thermometer for $10--it goes up to over 200 degrees, so it will read 180 degrees easily. I heated the milk in a saucepan over medium heat until the temperature registered 180 degrees--and I stirred the milk constantly. Once it hit 180 degrees, I poured it into the slow cooker (which had been heating up on low), unplugged it, and allowed the milk to cool (preheating the slow cooker is likely unnecessary in the height of the summer if you do not use A/C; in the cooler months or with the A/C going, it's probably a good idea to keep the milk warm). I checked the temperature every 20 minutes or so until I got a reading of about 116 or 117 degrees--then I took out two cups and stirred a half a cup of yogurt with them, and stirred that into the rest of the milk. I covered the slow cooker and wrapped the blanket around it.
Eight hours later--and I had perfect yogurt!
You can also read some tips about getting good yogurt here.