Another secret many chef's use is thickening the gravy with equal portions of thickeners made from both a Roux and a slurry made with cornstarch and liquid. The addition of the cornstarch adds a sheen to the gravy a roux alone won't give. Also, arrowroot does not impart a 'floury' flavor and need not be cooked. Adding too much of any thickener will make your gravy too thick. Add a little at a time.
Salt is a powerful and useful seasoning.
Use it as a last resort but don't be afraid to use it. A gravy leaning to the stronger or even slightly salty side will be much more welcome at dinner than a wussy flat gravy with no taste. Serve the finest gravy in your neighborhood. Your Friend will only be emailed an invite.