1)Don’t use the vegetables you used in making the broth in the actual soup, use fresh ones and cook them in the broth until they are tender.
2)Carrots in almost any kind of broth making IMO make it taste better. Parsnips are also great in a soup base, they help draw out the flavor, too.
3)If making chicken soup, get a fowl (old hen), they make the best broth. Don’t be afraid to use the gizzards and such in the broth, they make the flavor that much better.
4)When using meat bones, if not already cooked roast them first, it makes for much better broth then cooking raw bones in the soup.
5)I have found it is better to use a low simmer and cooking longer then using a rolling boil when making stock.
6)Get a good quality stock pot, the heavy enameled ones retain heat and to me they do a much better job.
7)With spices and herbs like bay leaf and the like, put them in at the end of the stock making process, don’t dump them in in the beginning; even better, use them in the finished soup, not the stock.
8)A couple of tablespoons of brandy or bourbon added to the stock towards the end of its cooking cycle adds to the heartiness and flavor (and the alcohol will burn off, not to mention 2 tablespoons in 4 or 5 quarts of water is not exactly going to have a lot of alcohol to start with).
9)If you make a big batch of soup stock at any point, freeze the extra in ice cube trays, it is really handy down the road if you need broth for something or making soup, you can even make a small batch of soup really quickly and easily.