There are a lot of good material on YouTube. Let me just share some videos that I found very interesting.
The first one is a seminar from B&H with Jerry Ghionis who has done a lot of educational material available through his webpage. He shows examples of settings that at first glance looks pretty ordinary, but by knowing what to look for he transforms them into beautiful settings for photos.
The second one is a great video about composition. The guy holding the seminar, Adam Marelli, started out as a sculptor and he is really good at breaking down and analysing photographs. It is long, but I can recommend watching the first 51 minutes.
Update January 2016, I want to keep this a living archive of good videos. Now adding a video talk by Sam Abell.
Updated September 2018. Saw this great video by Felix Kunze explaining feathering of studio lights.
November 2019. This video is interesting because it embraces imperfections. We constantly strive to make a picture a little bit more sharp, the colours a bit more vibrant, but maybe we are doing ourselves a disservice.
The dphog channel is amazing, he has several good videos about how to handle light and also a bit of post processing. Unfortunately it has not been updated for the past two years.
Nathan Elson takes amazing photos, and here he talks about how he feathers light. A short video that opened my eyes to how to better use flashes.
If you find any good videos, please share your recommendations.
Lastly, two books that I read and found useful are “Composition in Art” by Henry Rankin Poore (Amazon link) and “The Photographer’s Eye: Composition and Design for Better Digital Photos” by Michael Freeman (Amazon link). They deal with composition and how to balance your images.
Also, have a look at this blog post by Tavis Leaf Glover about why Rule of Thirds might be hurting your photography.