There are two ways to get your swing off to a bad start. One is to fan the club to the inside by rolling your left arm. The other is to lift the club straight up, as though you're trying to squash a bug on the ceiling with the clubhead. In either swing, your left arm separates from the left side of your chest, destroying your path, face position and coil.
CONNECT YOUR LEFTS: There's no simpler, more effective swing key than to keep the upper part of your left arm tight against the left side of your chest. Like magic, maintaining this connection syncs up your body turn and arm swing so that you stay on-plane and keep the club face square throughout your backswing. On the way down, make sure you maintain the connection. This, too, will help you stay on-plane and let you use body turn -- not just your arms -- to generate power through impact.
When you see a Tour player hitting shots with a towel or headcover tucked under his left armpit, he's practicing these moves. Try it in slow motion at first to groove the right feel [either with or without a headcover], then move on to faster swings. Your swing will instantly feel more compact and a lot more powerful.
"Connect" your left arm to your chest to keep the club on-plane during your backswing.