How to make good comebacks ??

Lots of people when I meet them at the beginning make fun of me and try to tease me thinking that I am really innocent person. I feel that my appereance (mainly my face) makes me feel that I am the guy who is gullible and can be made fun of. How do I combat such people with good comebacks. I don't want a list of comebacks, but the technique on how to do it .... Thank you :)