After the lumps are witnessed in the gravy, use a large metal spoon to try breaking them. The trick is to trap them under the spoon and press them gently.
Clumps are actually flour balls and most of them can be easily broken with the slightest of impact. Try to crush the lumps against the sides of the bowl and break as many lumps as possible.
Whisking the gravy can also prove helpful, especially if the lumps are small and not very stubborn. Whisk vigorously for a smooth gravy. A figure eight motion of whisking is an effective way of breaking the lumps.
If the above tip does not work, take out the largest strainer that you have. Place it over a bowl and pour your gravy through it. The sieve will hold back the lumps and let the gravy pass through. Make sure that the lumps are not pressed through the sieve. While pouring the gravy, do it slowly to avoid any run off from the top.
The third and most effective step is to use a blender. An immersion blender is most preferable, however, a regular one can also be used.
Pour the gravy into the blender and turn it on. Make sure that the lid of the blender is closed tightly as the hot gravy can cause burns in case of a spill. Blend for a while and check for the lumps. Blend again if the result is not satisfying.
If there aren’t any lumps left, pour the gravy back to cook. The drawback of this tip is that any cooking ingredients will also be blended with the gravy and the dish can lose its texture. The taste will of course be there.