Usually when I make soup I use water. I always use tons of vegetables, and I let them get nice and caramelized in the oil or butter when I'm sauteing them, so I figure, it sort of turns into its own stock, right? Plus, you know I'm a bean-cooking fiend, so I always have plenty of "bean stock" to round out the soup with flavored broth.
Sometimes one needs a good stock (I'm thinking particularly of the stuffing I made last month for Thanksgiving, or this baked dish with beans and potatoes I've been making from Mark Bittman's How to Cook Everything Vegetarian, or, even, sometimes, to add depth to soup.)
So, how to proceed?
I've been making one that's very close to Mark Bittman's roasted vegetable stock, where you roast the usual suspects (onion, garlic, celery, carrot, maybe a potato and/or a parsnip, if available), plus mushrooms, in plenty of olive oil in a really hot (450!) oven, till they get nice and browned. I de-glaze the pan with some white wine, then throw in several cups of water and a little soy sauce. Dried herbs or a couple of sprigs of fresh (parsley, thyme), plus a bay leaf help round out the flavor. I let it gently simmer for about an hour before cooling and straining.