Go back as if you were going to 'backhop' and just practice keeping the bike there, rocking backwards and forwards. Apply pedal pressure, and release the brake to let the front come up, the reapply brake, release brake and pedal, etc, kind of doing a halted wheelie. Keep practising these kind of braking wheelies, until eventually you get the bike to leave the ground by doing it (apply more pressure to pedal and brake) Easiest way though is to go riding with someone that can do it, and get them to show you...