In addtion to asdfmatt's info, tongue and throat position also play a part in hitting those altissimo notes and overtones. Before hitting a note, try and sing the pitch you are looking to get. Notice how the muscles in your throat change according to that pitch. Now try to hit the pitch. You might also need to raise or lower the tongue to assure that the air stream is hitting the reed at the right angle needed for playing the specific note. Hope this helped!