I make a lot of soups, especially during the cold season, which some would argue here in Alaska is every month besides July.
Making your own soup stock is really not difficult, and very rewarding. I have found that it does not take much time. Here's what I do:
Meat (chicken, beef, even fish): whatever bones, fat, or other trimmings, throw them in the freezer until there are enough to make stock. I keep a ziploc labeled "beef", another "chicken", and add to it whenever I trim something here or there. These are usually raw, but could also be the bones of a roasted chicken or turkey, or the barbecued T bones from T-bone steaks...
Vegetables (mostly roots, see "Suppengrün" below): I trim the end of the carrots, onions, etc., but that does not go into the freezer -- rather in a plastic bag in the vegetable drawer, instead of throwing them into the trash or compost. If I don't get around to making stock soon, and they get nasty, then the worms get them after all... On stock-making day, I also scour the vegetable bin for any limp carrots, celery etc., anything past its' prime!
In Germany, you can buy "Suppengrün" (literally greens for soup) at the farmer's market or in the grocery store. The French call it "mirepoix". It is usually purchased in a bundle and consists of a leek, a carrot and a piece of celeriac. It may also contain parsley, thyme, celery leaves, rutabaga, parsley root and onions. The mix depends on the region, but they are usually cold climate roots and bulbs with long shelf lives. The website Germanfood.about.com proclaims that Suppengrun vegetables and herbs "impart hearty, strong flavors to the soup or sauce, which makes them perfect foils for other strong tasting ingredients such as dried peas and beans or pot roast." In other words, my husband would say, anything to make lentil soup more palatable!
Here's my recipe:
Homemade Soup Stock
Stockpot full of cold water
meat scraps, frozen or thawed (optional) -- skip if vegetarian
celery root or stalks
Do NOT salt at this time! Wait until stock is done, or better yet, until making soup.
Bring to boil, then reduce and let simmer for several hours or all day (the big turkey carcass after Thanksgiving might go all night...). Length of time depends on the quantity and esp. the type of meat: beef goes longer than chicken, fish only 1/2 hour or so. Let cool until you're comfortable handling it. Pass through a sieve and fill in glass jars with lids. Refrigerate. A layer of fat will form on top, which acts as a seal. These will last several weeks in frig, but if you do want to freeze the stock, then remove fat and decant stock into plastic containers (leave enough headroom for expansion).
Hint: concentrated stock can be frozen in icecube trays, then transferred to ziploc bag. Then if you need just a little bit of stock for a recipe, grab an few stock-icecubes.