I think the largest mistake a beginner makes fly casting is breaking their wrists. Rod hitting structure behind you, snapping off a few flies, rod making some horrific noises? These are all signs you are breaking your wrist too much.
You arm is the piston that drives the rod and generates the power. Your wrist simply guides the cast. This is a basic concept that is going to enable you to cast better. Most folks; however, don't really recognize that their wrist is breaking to begin with and the need to keep it locked.
Need a couple of tricks to help? Get a large, thick, rubber band and put it around your wrist. Tuck the butt of your rod under and start casting. You can feel what is going on with your wrist by the stretch in the rubber band. The more you break your wrist, the more tension on the rubber band. The goal here is to have as little as possible so you develop the technique of casting with your arm and not your wrist.
Tight Lines and Screaming Drags my fishy friends!