You know what I love? Powdered milk. Gross right?
But hear me out. I don't drink milk - the only time I buy it is to make yogurt - so buying even just a pint to use for the occasional recipe that calls for a cup (quarter cup, splash, tablespoon) of milk yields at least half a pint of spoiled milk. Solution? Powdered milk. It's right there in the baking aisle, and it's super cheap. Really, it is! It seems expensive up-front because you're going to drop $6-9 depending on how big a box you're buying, but you're getting gallons of milk out of it. And it doesn't spoil. And you can bake with it, and make French toast with it,make buttermilk with it (1 T lemon juice/cup), and, apparently, drink it (though I've been told you should chill it very well first).
This is the reason I bought it. I'm almost done with my first box and bought a second last time I was at the store; this time I went with the pourable variety, which I would recommend unless you're planning on making large quantities at a time (the other option comes in packets to make a gallon).
The Hillbilly Housewife - This is a cool website in and of itself, and it provides plenty of uses/ideas/conversions for powdered milk.
101cookbooks yogurt - I don't do my yogurt exactly this way, but I throw some powdered milk in when I think about it.