There are times and situations that teasing someone can create a bond of friendship and that the bond created is a result of a well-meaning intention to prepare a person for what they might encounter in the future.
I don't mind the occasional teasing, but there are cases when the teasing goes a bit too far and begins to sort of sting. It's not because they are intentionally being mean, but rather that I'm a variable person and can be rather sensitive in certain subjects.
Personal appearance for example being the biggest one I can think of, I already am well aware I look like a werewolf, and have done my best to mitigate the look with no success. When I've spent an hour that morning working on my appearance to avoid this, and someone pokes fun to try and get me to smile, I usually plaster a fake smile and join in, just taking the hurt.
To be clear, I don't want to make them feel ashamed, and I have fun (most of the time) joking around with them and teasing each other. Some subjects are particularly hurtful to me personally though, hence the question. I don't want to lose the teasing dynamic I have with them, but if I bring up the subjects that are painful to me, they'll avoid it entirely and I might lose that bond I have with them.
How can I tell someone their teasing is hurting my feelings without being seen as too sensitive?
Edit: This applies to family (extended like cousins etc.), and coworkers. Those that I choose as my friends know where I'm touchy.