I have had the same problem as forwardthinking... probably thought too much when I should just be enjoying the conversation. I can still recall this running commentary in my head where I'd think of something to say, then analyse it, then think it wasn't that clever, and by that time, the conversation would move on and I'd start the whole cycle again. It seems ludicrous, I know, but that's the way it was (and sometime still is, to be honest). This was coupled with mumbling, so at social gatherings I'd often feel like I wasn't even there - even when I did have something to say!
It's not quite the same as just being shy; it's more a matter of being too self-aware or self-conscious to participate in a collective discussion. (Yes, I know: we should just try to live in the moment, without self-regard, but this is a very hard thing to teach yourself.) I find that parties and other large social gatherings are more about exhibition or acting, than they are about conversing... but that's another issue...
Anyway, what helped for me is that I worked as a waiter and a barman for about 5 years while getting through university. I was continually facing new people and needing to build a rapport with strangers. I had to present myself well, and that includes having good articulation. The other thing is that, in being in a different environment as a waiter, I could use this to focus on my speech. I still catch myself out talking quietly from time to time. But it's nowhere as bad as I used to be.
I'm hardly Orson Welles, now, of course. But I'm better than I was. So, if you're wealthy, there's vocal training and voice teachers. If you're poor, do some bar/waiting (and all the while, tell yourself it's about the speech improvements, not the crappy work conditions) and practice projecting your voice. And as the man said - "Express yourself!"
