Did you know that you only ever need to buy one carton of plain yoghurt? (provided you eat it most days)
Boil some fresh milk (it doesn't work as well with UHT or skimmed), then let it stand until it's luke warm. Put it into a tub and add 1 or 2 tablespoons of your original plain unsweetened yoghurt, stir it in.
Leave the tub undisturbed overnight then put it in the fridge to chill.
When you make it again use 1 or 2 tablespoons of your new yoghurt to add to the luke warm milk.
And so on...........eternal yoghurt!
(By the way it's really good with maple syrup)