Find the sweet spot for balancing the ball on your forehead. This is usually right around your hairline. Start around there and either place the ball or lightly toss it onto your forehead and try to balance it for a couple seconds.
Practice juggling with your head. You can start by holding the ball in your hands, tossing it into the air a foot or two above your head, and trying to keep the ball up over your head. Stand with your head tilted back and your neck relaxed, and keep your arms out away from your body for balance. Make sure your eyes are focused on the front edge of the ball, your knees are bent slightly, and you are on your toes-- not flat-footed. Keep the ball up by hitting the ball in its center with your forehead, and don't just let the ball hit you in the head--push up into the ball slightly with your upper body to keep the ball up.
Lighten the touch on the ball as you juggle on your head. As you reduce how hard you are hitting the ball, the bounce off your forehead will get less and less. When the ball is only slightly rebounding off your head, you can then cushion the ball at the moment of impact by lowering your body slightly and catching it on your head. By being on your toes with your knees bent and your arms away from your body, you will be able to adjust and keep the ball on your head.
Try juggling from the ground to your head. Start with the ball on the ground in front of you, use the bottom of your toe to pull it on to your foot and begin juggling with your feet, eventually popping it up into the air above your head. You can either toss it up just above you and try to catch and balance it on your head right away, or put it up higher and juggle with your head before trying to catch and balance it.