My first batch of yoghurt came out kind of lumpy and gritty. The taste wasn't bad, but the texture was off. A quick search of The Internet tells me that this is the result of too high temperatures. It makes the milk curdle.
This afternoon I decided to try again. I boiled up some cheapo UHT milk (I hear that even soy milk will work), let it cool down a bit more than the other day, and poured it several times through sieve in case any lumps had formed during the boiling. I stirred in the last two tablespoons my lumpy home-made yoghurt, poured it into jars, and placed the jars into a bucket of - not hot - but pleasantly warm bath water. This time I wrapped the whole bucket in aluminum foil and a big fluffy bath towel.
That was about seven hours ago. I just checked on it. The yoghurt's smooth but still a little on the runny side, so I'm going to let it sit for a couple more hours.