Well-written fiction gave me templates in "language" I could understand. To make character emotions really hit home, authors will usually describe the same one from several different angles: here is what happened to character X, here is something the character's face is doing, here is how character X feels, here is what X is dwelling on about the situation, here is how X behaves. I read extremely fast, so absorbing lots of decent books let me build up pretty good libraries of correlations between such things; then I could guess-and-check with this narrowed search space in the real world.

I learned my social skills mostly through observation and trial and error. I'd watch other kids in school, see what kinds of body language they used in specific social and emotional contexts, and try to figure out how other people reacted. Ditto for tones of voice and word choice. I also spent a lot of time mentally mapping out social relationships between my peers in as much detail as I could and tracking them over time, but I don't think that part yielded too much in the way of useful skills or information.

Whenever I thought I had some small piece of the puzzle, I'd find a context in which I could try it out, fail horribly, try to figure out what I did wrong, and try again. Once I'd realized that the only common element in all my social failures was me, it took me maybe 2 years to hit a level of basic competence, with gains coming slower after that.

I also benefited from bouncing ideas off of a therapist on a regular basis, though I suspect any sympathetic adult would have been about as helpful; the most useful part was being forced to clarify my thinking in order to coherently explain it to someone else. And having a low-pressure test environment didn't hurt, either.

A couple disclaimers:

• This approach worked for me, and it has worked for some other autistic-spectrum people. (I have Asperger's, FYI.) It may not work as well for you, or at all.
• If it will work for you, this is likely not the best, fastest, or easiest route.
• The brute-force approach is hard and unpleasant. You have to be willing to fail over and over in what are often highly embarrassing ways.
• Rehabilitating your reputation in the social circle(s) you use for practice will be all but impossible.

Hope that helps.

Asperger's diagnosis here.

My advice to someone in my 17 year old self's position would be to get a stack of social psychology books and read them, along with succeedsocially.com and a few ettiquette books that explain the why as well as the what. Then find a venue in which to practice those skills, whether it be work (bar or restaurant work at best because of the continuous exposure to people) play (an interest group, like a club or society). If you choose play it must actually be fun because otherwise you will not go after a while. Something like comedy or acting would be great as would dancing or martial arts. I suspect the PUA community would be excellent training for men who already have training wheels but I have no personal experience with it, I just found the books helpful in providing me with better mental models of people.

My personal experience.

Alcohol fueled disinhibition leading to expanding the range of behaviours I was willing to try, and non-deliberate practice by doing more of the things that seemed to work and less of the things that didn't. This was one of the major things.

Other important ones;

Discovery Channel programmes on flirting gave me a base on which to build with interacting with the opposite sex in a more natural or at least more fluid and comfortable (for all concerned) way.

Working and going to university also made a huge difference. I was more reserved than was helpful before. Dealing with extroverts and non-intellectuals all day at work was really helpful un rubbing off some rough edges and university was great for dealing with a group with common interests and varied levels of social skills. You could see what worked and given that it was an SF society there was a large spread, and some of the people there had made large strides very quickly, some of whom I became good friends with and who were fonts of good advice.

Lots of autistic children get training by, for example, studying cards that show people with different facial expressions and by playing "social detective" games in which an adult asks the child to figure out what a person is likely feeling in a given situation.

In the future there might be wearable machines that surreptitiously examines faces and bodies of nearby people and transmits an analysis to the machine's wearer. PUAs would likely drive the market for these machines.

"Hard, bitter experience" might sound like a pithy response, but it's also an honest one.

My hypothesis (I e-mailed Simon Baron-Cohen and he agreed) is that autistic people don't subconsciously "synchronize" with people in the same way that neurotypicals synchronize with each other. "Synchronization" seems common in the animal kingdom - this is why spinner dolphins can be extremely good at coordinating their movements together without even having to communicate at all.

So autistic people have to learn these signals very consciously, as explained in all these other posts. And conscious learning is often clumsy (which often means oscillatory convergence around a fixed point)

Positive:

Enjoyment/Joy - smiling, lips wide and out
Interest/Excitement - eyebrows down, eyes tracking, eyes looking, closer listening

Neutral:

Surprise/Startle - eyebrows up, eyes blinking

Negative:

Anger/Rage - frowning, a clenched jaw, a red face
Disgust - the lower lip raised and protruded, head forward and down
Dissmell (reaction to bad smell) - upper lip raised, head pulled back
Distress/Anguish - crying, rhythmic sobbing, arched eyebrows, mouth lowered
Fear/Terror - a frozen stare, a pale face, coldness, sweat, erect hair
Shame/Humiliation - eyes lowered, the head down and averted, blushing