Make a basic cream sauce by melting margarine over moderate heat. Stir in flour; keep stirring until flour and margarine are combined and smooth. Remove from heat and add the chicken broth and milk, a little at a time, stirring to keep lump-free. Return to heat. Bring sauce to a gentle boil and cook, stirring constantly, until it thickens. Taste and adjust seasonings. Can be varied by using vegetable broth, sautéed diced celery, celery seed, or sautéed diced mushrooms.