Walk and Move in Rhythm
When performing on the harmonica (formal or informal setting) you're often standing, making the standard tapping of your foot a bit awkward. Tapping your heal works a little bit better while standing, though this still assumes that you're staying in one spot.
When on the stage it's much more pleasing to be moving around a bit... it shows you're feeling the music. To move around in rhythm requires that you have a bounce in your step... knees bent... etc.
Experiment by listening to one of your favorite songs while standing, shifting your weight side to side, front to back, stepping, etc. Experiment with different movements so that you can keep the beat in your body. Then try the same thing while playing your harmonica to a song you're very comfortable with. I know it sounds simple, but until you've experiment with it, you'll feel and look uncomfortable on the bandstand.