My kefir culture (bought from a neighbor on Craigslist!) is no longer usable. The active culture went bitter somehow – probably contamination; the backup culture in the freezer died, apparently. I tried to make several batches of kefir with it, hoping for a revival, but nothing happened 🙁
Although kefir is easier to culture than yoghurt since it is mesophilic (grows optimally at room temperature), I prefer the flavor and texture of yoghurt. Unfortunately, yoghurt cultures are thermophilic (grow optimally around 110°F). It is difficult to maintain this temperature without special equipment (yoghurt maker or water bath) or lots of fiddling (e.g. with oven settings) for the 5-10 hours needed to make a batch of yoghurt. A recent read gave a good tip for getting around this: put the heated milk and starter culture into a thermos. Even bad thermoses are capable of keeping their contents hot for 5 hours, so it works out perfectly, no additional energy input necessary!
I tried the thermos approach with a squat, wide-mouth thermos from Goodwill ($2.19! Stainless steel! Thermos brand! Oddly, it is branded with images from some car movie). It worked really well! The yoghurt turned out thick (even thicker than the commercial variety I used as starter) and creamy. I will definitely do this again!