I made a conscious decision at the start of this season to improve my casting. I didn't have any specific problems in mind but I just knew I could be better and decided to do something about it.
My approach was to go back to basics and work on things like loop control, casting plane, accuracy, controled tailing loops etc. I also decided to spend time learning to cast left handed.
There is a non-commercial site called "virtualflycasting dot com". Its actually dedicated to resources for casting instructors but it does have a lot of interesting stuff. The Federation of Flyfishers site has a number of recommendations on books & videos.
When sight fishing, look over your shoulder from time to time, you never know who's behind you