Improving Social Skills
Social Communication by Dr. Mary Shapiro. This site covers topics I have addressed here, and more, with detailed examples, exercises and videos.
Improve Your Social Skills by Daniel Wendler
Succeed Socially website by Chris MacLeod
Communication Skills by Dan O’Connor (video playlists)
A Toastmasters club is a great place to practice communication skills.

Maintaining Conversations
Conversation by Daniel Wendler
How to Socialize and Keep a Conversation Going on engVid (videos)
More recommended short videos here and here.
How to Make Conversation articles by Chris MacLeod
How to… Conversations on the BBC. Audio files on how to extend conversations, end conversations, respond to compliments, show interest, and more.

Nonverbal communication
People don’t restrict their attention to words – they also observe. The messages words convey can be helped, or hurt, by the messages the body sends. To be successful in communication, it helps to be mindful of body language.
What is Nonverbal Communication
Good and Bad Example of Body Language (a short video)
Body Language Do’s & Don’ts by Faye de Muyshondt (video)
Is improving your life as easy as changing how you sit or stand? by Eric Barker
Learning To Make More Eye Contact With People by Chris MacLeod
How To Make Eye Contact by Daniel Wendler