Concentrate on viewing the image not as a face, but as an everyday object. According to a 1979 University College London study, humans are better able to recognize inverted images of inanimate objects, but have a difficult time doing the same with faces.
Memorize the individual features of a face if you want to recognize a face while it is inverted. Independently focus on what the eyes, nose, mouth, ears, hairline and chin look like. If possible, separate each element as you study it by blocking off the rest of the face, as this may aid you in recognizing it later.
Look for distinctive features such as a bald head, facial hair or glasses. These elements are easier to recognize than other facial features.