I know some like the reliability of a yoghourt maker, but this is my method:
I buy Yeo Valley yoghourt, maybe once every 4-6 weeks. Inbetween I do this:
Heat 500ml. semi / skimmed milk to 82 degC.
Whisk in 3 tablespoons dried milk to thicken it
Cool to 43 degC - I do this whilst doing other cooking or kitchen tasks, so not onerous.
Whisk in 3 tablespoons yoghourt.
I rinse out a wide-neck thermos with boiling water to warm and sterilise the thermos, then pour in the yoghourt mix and leave for 12-15 hours.
You can keep doing this for quite awhile, but eventually you may find the yoghourt gets tainted - that's why I buy a fresh yoghourt when I do a Co-op shop.
As I have a thermometer and wide-mouth thermos to hand, and use dried milk to make porridge, this is easier for me than having another gadget - it may not be the same for everyone.