Play around with it; keep your teeth, lips and tongue out of the way; try breathing in and out a bit during the notes, changing the size of your mouth cavity and pulling your lips back in a sort of grimace for more thin sounds occasionally; keep the tines of the jew's harp firmly against your front teeth, not between them and don't bite down. And develop a sort of "strumming" rhythm with your twanging hand. Practice.

By the way, different sizes of jew's harps will produce different ranges of notes. You can't use one single one for all keys (if you care about that sort of thing... some people don't).