Another way to prevent lumps in the first place is to pour some of the broth or other hot liquid into a mug or bowl and add your flour/cornstarch to this reserved portion. Combine the mixture with a fork and take the pot off the heat. Whisk the mixture into the rest of the hot broth and return it to the flame, whisking all the while. This helps your flour integrate into the gravy and thicken it, without creating lumps.