Practice! Also group crits. While the format differs from place to place, in art / CGI / illustration / animation a crit (critique) is a session where everybody lines up their work and then you go around the room and each person critiques each others work, saying what you like about it, what you dislike, what you would change etc., while the person being critiqued takes on board or pushes back against the criticisms. Because you're taking it in turns to critique and be critiqued, and are doing it in person, there's social pressure to be polite and constructive, however it is still very uncomfortable to begin with. But do it every week for a few years and you end up with a very thick skin. You can take lessons from the process into other areas of your life and even introduce something parallel when developing where you do group code reviews.
