I make low fat yoghurt for my dog but prefer full cream milk for mine as it is nicer and thicker. Low fat milk tends to make a runnier yoghurt. I used to put a bit of powdered milk in my yoghurt to help it thicken more but then I read that powdered milk has a lot of cholesterol in it so I stopped doing it. I find that if I hold the milk at 180F for 10 minutes before cooling it I get a thicker yoghurt than if I get it up to temperature and then cool it straight away.
I have heard that you can use food dehydrators to incubate your yoghurt.