In principle, making yogurt involves giving a 'good' bacteria culture the ideal habitat in which to breed. In this case, they are a little colony of lactobacillus whose babies like warm milk. What baby doesn't like warm milk? So theoretically, infusing warm milk with bacteria colonies and maintaining an ideal breeding temperature for the little critters is pretty simple.
I used to make yogurt in a tupperware and then keep in in an esky/cooler packed with bottles of hot water to maintain the temp. I was finding that the results were a little irratic, depending on how diligently i was maintaining the temperature. There are lots of variations on the internet, using ovens, crockpots, piles of blankets etc.
I was originally turned off by the 'Easiyo' yogurt maker because it is actually meant to be used with prepackaged powder mixtures containing the bacterial culture and powdered milk - however these are not as cheap as just buying your own milk and reserving a little yogurt from a previous batch as a starter culture. Anyway, i invested $20 on an Easiyo yogurt maker a month or so ago - it maintains the idea temperature really well, and now that i know i can easily make yogurt without the Easiyo packets, i haven't looked back! Wow i sort of sound like an Easiyo info-mercial (except on the website they highly DO NOT recommend using milk in a DIY fashion, claiming you cannot guarantee product safety or something).
I've also read that your 'home' culture takes on a little identity of its own and is somehow better for you, but i haven't confirmed that scientifically. I do notice that if i accidentally don't save any yogurt for my next batch and have to buy a pot of yogurt it takes a few batches for the flavour to mellow. Usually the first few have quite a sharp sour flavour in comparison to yogurt made with the following 'generations' of starters.


