Not unlike habit formation, a significant part of this ultimately comes down to repeatedly telling yourself, "I am not going to be the kind of person who freezes up in social situations" until you eventually alieve it.

This may actually require going to parties or other crowded situations and just standing around quietly the first few times. (As someone else once recommended on LW, working out is good in conjunction with this -- people are more accepting of the strong silent type.) Sooner or later, you'll luck into hearing a conversation you can't help but contribute your two cents to.

Additionally, for me personally, OKCupid has been a huge blessing. I have much easier time approaching the opposite sex in writing than in person. But once you've done it in writing enough times, doing it in person seems much less intimidating.