The best advice is to make sure to have a 'harp with as light or thin a tongue as possible. Rest the tines tight against your teeth. Gently place your lips against the tops of the tines. Hold the curved part firmly, avoiding the tongue completely. Keep your tongue back, but to get different notes you'll tend to move it up and down. Pluck away.
Keep on the practicing, the jews harp is a fine instrument a noble instrument that few understand and few fully appreciate. Once you get the hang of it you'll be ready to join the Jew's Harp Guild (check the web and there's also the address around here somewhere)