Simmer longer to reduce and thicken
Mix cornstarch with cold water (or broth), then stir into simmering gravy
Use flour slurry: mix flour with cold water, then stir into simmering gravy
Make a roux: cook equal parts fat and flour, then whisk in hot gravy until smooth
Use instant mashed potato flakes to thicken
Stir in a small amount of butter and flour (or butter and cornstarch), whisking until smooth
Use arrowroot: mix arrowroot with cold water, then stir in and simmer briefly
Use gravy thickened with gelatin: dissolve unflavored gelatin in warm water, then whisk in
Mash in cooked potato or blend a small portion of gravy with solids, then return to pot
Whisk in a small amount of cream cheese or sour cream for thicker, richer gravy
Strain out lumps, then simmer again to set the thickness
If too thick, thin with hot broth or pan juices, whisking until smooth