One of the things that annoys me about our culture is that it assumes that everything is a one-off use – especially in cooking.
Mainly I’m talking about yeast. Yeast isn’t like a whole bunch of seeds that grow until you’re ready to use them and that’s it… yeast is a micro-organism! This means that you don’t have to buy a lot of yeast if you’re going to be doing a lot of baking in a short amount of time.
Just double up the starter solution (usually water, sugar and salt… occasionally flour) and add the yeast – wait a bit longer and there you go! Keep feeding it sugar-water and you could have one batch of yeast last, well, forever.
Make sure to stir it, though – there are molds that feed on yeast if left still for too long.
Oh! And you can do the same thing with yogurt! Grab some plain (or flavoured, I don’t think it really matters) yogurt and add a dab to just-boiled-and-cooled milk. Leave it in a relatively warm place and you’ll have unflavoured yogurt in days for just the price of milk!
If you’re wondering why I recommend boiling the milk it’s to kill off anything that would compete with the yogurt-making micro-organisms (I think they’re a bacteria)