This is a good tip to check your swing plane on the back swing. Its a great guide to check you are starting you back swing on plane.
Swing your club back away from the ball normally and stop when the shaft is approximately parallel to the ground. The butt end of the grip should be pointing directly at the target. If it is pointing left of the target you are swinging outside the line and will probably slice. If it is pointing right of the target you are swinging inside the line and may hook it.
The position the club is when you have it pointing straight down the target line is exactly where the club should be on it’s way back down. It is easy to allow the club head to stray outwards (fade/slice coming) or inwards (draw/hook coming) on the way back down. it is however, easier to achieve if it passed through the correct position on the way up.
If either of these faults are there on the way up, return the club head to the ball and repeat until you are achieving this correctly. It is not a guarantee that you will stay on plane for the rest of your swing but it is a very good start. Make a mental note of your arm, elbow and hand position when the shaft is pointing down the line and try to get your arms, elbows and hands to move through that position on the way down.
It helps to regard the swing as a single event as opposed to to a back swing and a down swing. By having this mental image in your mind you have a much better chance of staying on plane. Don’t get too obsessed with swing plane in your swing. It is important, but look at Jim Furyk and see what he can do with two different swing planes.