I’m a big fan of Pete Kutzer, I think he is one of the finest casting instructors I’ve ever known. He’s put together an exceptional video of the five most common casting mistakes. He talks you through what the mistake is and then gives you pointers on how to fix it.
This is a video you need to bookmark so you can quickly refer back to it when you casting needs a bit of a tune up.
Check it out at Fly Fishing Video Lessons – The Basics of Fly Fishing – Chapter Eight: Reading The Water – Correcting Common Casting Mistakes.