We strongly recommend what we call a "set-up waggle" (or shoulder waggle). It's something like a Mike Weir mini backswing. You make a slow one piece backswing only up to the point where the butt of the club points at the ball. Then you come back to address.
During this mini warm-up swing you check to see that you are on-plane during the backswing by CLOSELY WATCHING THAT THE BUTT OF THE CLUB POINTS AT THE EXTENDED TARGET LINE AS IT APPROACHES THE TOP OF THE BACKSWING and returning to address on the downswing. When you practice the Set-Up Waggle it's a good idea to put a tee in the butt end of the club to make it easier to see make sure the butt end of the club is pointing at the extended target line.
BTW, the target line goes from the target back to the ball. The Extended Target Line is an extension of the target line back behind the ball.
Two check points during the backswing. Check point 1. When the club is 45 degrees back (from address position) you should see the shaft pointing at the Extended Target Line (then you know you're on-plane).
Check point 2. When is at 90 degrees back from address (when the club is horizontal) the shaft should be parallel to the Extended Target Line.
At the top of the Set-Up Waggle it's a good idea to stop then turn your head and look at the position of your front arm and the club. When your front arm gets to horizontal on the backswing you should check and make sure it's parallel to the extended target line
Also check to make sure your lead wrist (left wrist for a right handed golfer) stays flat. Make sure your are turning your shoulder by making sure you maintain the triangle formed by your chest and two arms. This is mostly a core (shoulders, chest & abdomen)turn so there not a lot of wrist cock (thumb cock).
Repeat this "shoulder waggle" a couple of times (2-3) to build confidence so you know you'll be on-plane in your real swing. Doing the Set-Up Waggle teaches you what it "feels like" to be on-plane.
The Set-Up Waggle loosens you up and it helps insure that you will be "on-plane" during your backswing and downswing. Many golfers report that the set-up waggle really help them a lot mentally because they feel more confident about making a solid on-plane swing.
I'd highly recommend this procedure on all shots but especially on tee shots. It doesn't take a lot of time to do this so it won't slow you down.