We definatly had that problem Ozzy! Yoshi loves shoulders too, and really we both like having her up there, that way she can go where we go but we still have free hands. At first she tried that biting, same thing, not aggressive just curious. She would nibble a bit then bite down and it would hurt. We started telling her when she nibbled "Be gentle!" or "Be carefull!" and if she bit down we said 'No' and removed her to her cage and left the room. 5 minutes later I'd come back, get her out, talk to her, not let her on my shoulder for a little while, then put her up there and say "Be gentle!" and she would be all happy to be up there again. If after a couple minutes she bit again... then the process starts all over again.
Believe me it's frustrating at first, you do it several times a day and then you think they understand and then the next day it happens again. But she bit for less than a month and I have never had an ear or face bite since (that's like 8 months ago) so she has been trained. Of course you could always ban her from your shoulder if you need to, but for us this was the good option Especially if it's curiousness, they soon realize that their beaks upset you. If Yoshi ever gets moody when I remove her from my shoulder, she is done with the shoulder for the day, but that rarely happens anymore. She is a pretty gentle bird
Hope this helps!