I always toss some celery and onion in ... I save celery and onion trimmings (not the roots, of course ) specifically to add to broth. I just put it all in a freezer bag, along with the chicken bones and wing tips. I store the bag in my freezer, and when I'm ready to make broth, I just dump the contents of the bag into my soup pot and add water to cover. I usually add a quartered onion, too, with the skin. I don't add carrots anymore, although I used to. I actually don't notice any difference in the flavor of the broth when I make it without carrots.

Sometimes, if I feel like going to the trouble, I make a bouquet garni for the herbs and spices.