I've done a few courses through specific business support groups (private consultants) and also through the organisations I've worked for both internally and through being sent off for training to private trainers. I've also read some books as well, probably more about communication two of which have stood out to me are How to Win Friends and Influence People (Dale Carnegie) and Men are from Mars and Women are from Venus (John Grey) which is a relationships book but one I've used to enhance communication. Also, some books on body language like The Definitive Book of Body Language (Barbara/Allan Pease) and The Dictionary Of Body Language (Joe Navarro). It may seem odd to cite these books but through better communication and understanding body language (others and my own) it's helped my professional career, public speaking, leadership and other things.
These books, together with the face to face training has helped me so much. Also, I've seen and met people (seen them speak) like Zig Ziglar and Niik Stewart to name only two, who I learned from through observation and 'osmosis'.
As for the techniques I use...See the books and training above, although observation is very important. I seem able to see tings about people that give me valuable information that I then use to gain an advantage, build rapport and relationships and value. I listen too, that's super important. Listen to HEAR not listen to RESPOND. Most do the latter and then respond to things they haven't listened to and understood completely. Truly seeing and listening is so important to communication and to moving things forward to better results, whether professional or with the ladies in the bar, you know? Most people are bad listeners.
Then, talking...we have two ears and one mouth and we should use them in that proportion. People talk to much, people like to talk about themselves too. The best way to build rapport is to talk less, ask more questions and be completely interested (genuinely) in the answers. It works with the ladies, at a date, with family, spouse and colleagues, and it works when in professional meetings and public speaking.
Smiling helps too, it's disarming and goes a long way towards making people feel at ease. A genuine smile I mean...but it has to be used at the right time, not when briefing a bunch of operators about to board a helicopter to assault a Taliban stronghold. You know?
Anyway, I'm sorry...a long answer, but it's a big (massive) question so I felt I had to give you a little more.