Practice. (Ugh, how helpful... :) )

For me, travelling outside my city and country lead to the biggest improvement in obtaining the courage of which you seek.

It boiled down to putting myself in a situation where I could go, "What's the worst that could happen? I'll probably never meet these people again in my life, so, so what if they end up thinking I'm an idiot?".

Of course that's ignoring the risk of viral video and retweeting which weren't really an issue back then. :D

The interesting result was that, if, say, I was in a group and asked what I worried might be a stupid question, but asked anyway, I'd have people come up to me after who would say "Hey, thanks so much for asking, I wondered the same thing, but was too worried/shy to ask".

Then I could revise my self-talk to "Hey, I'm probably not the only person wanting to ask this question, so it's okay for me to do so".

Also, spend less time listening to yourself tell you why you shouldn't. And go ahead.

Start now! Turn to the person nearest you who you don't know and start a conversation. Then come back and tell us how terribly it went and how you would've been better not to have started it.

Or, you know, if it goes well. :)