Summarize the following:
Try performing on a stage that is raised above the audience, or turning at a 45-degree angle in the opposite direction. Figure out which angle is most convincing given your particular skills and body type.  You can also control the angle by controlling where the audience sits. Try positioning them at different distances from the stage. You can also deliberately block the audience's view as is done in the Superman or King levitation, where you place a jacket in front of your feet while setting up the illusion. Mimic the expressions you would use when lifting a heavy object with your muscles. Have a look of concentration on your face. Use body language to further convince them that the levitation is both a mental and physical exertion on your part.  Widely known magician David Blaine even pretends to be sick after performing the levitation, further convincing the audience that he exerted tremendous energy during the trick. Pretending to be surprised or shocked that the levitation worked can also be convincing.

Summary:
Work from different angles. Act as if the levitation is difficult and intense.