It's all about the Grip

Here's the proper fly rod grip. Place your thumb on top of the grip, then wrap your remaining fingers around the cork. Do not put your index finger on the top of the handle, this will not help your aim or casting ability.

Practice casting with this grip from the very beginning. Your aim will improve, and you'll have much more power for longer casts.

Another tip: don't bend your wrist when casting - Lock It!