Ultimately it was exposure and practice of various kinds over the years, some natural some "enforced/intentional". I am a strong introvert (defined here as: I love people but social interactions exhaust my energy and I need to recharge alone). I used to be completely quiet in social settings during ages, oh, 6-25 or so, and very definitely shy & awkward. Today, people who have met me in last decade are convinced I'm a chatty extrovert.

Few things:

1. In my mid-20's, I got a consulting job. I went from project to project, meeting new people, team members and clients. Building from a central core of confidence in my competence, it "leaked" that confidence into the accompanying social interactions as well. I dealt with people younger and older, technical and functional, of all kinds of personalities and walks of life. But! We had a reason to be there, a goal and mission and process and procedure, so it eliminated a lot of awkwardness and uncertainty and exposed me to varied social interactions 40 hours a week for years. As I grew my confidence in my work skills my confidence and comfort in social interactions grew as well.

2. I was a shy child, then in a civil war during my formative years of ~12-15. Let's call me a "late bloomer" when it comes to opposite sex :). After years of awkwardness and "lusting from afar" and just not going anywhere, I finally blew my gasket and one day decided "Enough is enough, I'll figure it out" and signed up for lavalife. I went on dates with conscious approach of "I'll probably survive this".

Following sentence will make different amount of sense and obviousness depending which side of divide you're on, but eventually I learned: It's not rocket science, it's not deadly, it's doable. If you're a honest, open, semi-interesting person, dating is not hard. (Finding "the one" is more and longer work of course; I'm lucky to have done so and happily married with 2 kids now :)

3. I have many interests and hobbies, so going to photography meetups and classes, rock climbing, motorsports, etc, have all allowed me to experience additional social interactions built around a core interest and confidence and competency. Volunteering to teach or help with something you're strong at (computer literacy for example) will do absolute wonders. Teaching is amazing btw - there's a clear and easily understandable social situation where all attention is on you but in a friendly and well defined way. And though that sounds Sheldon-like, it is also amazingly rewarding fun and interesting!

Also note, there are many paths to social acceptance. I have not chameleoned fully into social norms and strive for middle of bell curve - I'm still considered a bit weird; but it's said in a loving / amusing way (usually :-). I own it, I have significant confidence into who how what I am, and that seems to be the key ultimately, at least for any people I am interested in :-).

[Personally, FWIW, and not to discourage but to share in wonderful variety of humanity :), Gym is the absolute positive LAST place I would've ever suggested my teenager self to reduce social awkwardness; for me, instead of starting from a core of confidence, it would start from the core of insecurity and incompetence, and to me a strange and incomprehensible social habits and contracts, suffused with body exposure and smells and all too frequently macho bravado and bro culture; but it may well work for others, and certainly if you do gym hard and long, it'll improve your body and body image (neither my confidence or self-consciousness have ever stemmed from my body; I'm neither embarrassed or proud of it - it just is :). ]