Some rambling thoughts on the topic:
I second Scafool's and other's advice (though the 11-to-1 can change a bit with different lines/rods and wind). When I have been away from fly fishing for a while and need to get my cast back, I find it helps to look back and watch for the line to just about straighten out behind you and then start the forward motion.

I would also say to practice in places where back-snags won't drive you nuts before you even get to a stream.

Beyond taking a lesson, I'd say go fishing with a guide. You'll get the casting lesson plus a lot more and it will be more enteraining than just a casting lesson. Of course, it can cost a lot.

As far as books, along time ago, there used to be a book -_ actually it was almost a comic book -- called the Curtis Creek Manifesto. It is fantastic. It's still available on Amazon for $10 and doubtless can be found used for less. This book covered all the basics in a funny easy to understand way.

On a more intermediate level than Curtis Creek Manifesto, I would suggest Prospecting for Trout by Tom Rosenbauer.

I've also read Tactics on Trout by Ray Ovington and it was good but it's for someone who wants to approach each possible boulder or riffle with a complete plan of attack. Still, it can be educational even if you just read a couple of chapters.

Finally, while I haven't seen it, as I was looking to see if Curtis Creek Manifesto was still around, I noticed on Amazon "Joan Wulff's Dynamics of Casting... (DVD) for $22. It seems highly rated...