I don't post a lot of in-depth instruction about swing plane. It's not that plane isn't important -- I just think we're too obsessed with it, and most of the drills are too nitpicky. But I don't mind putting up something that's both simple and useful.
GCA Coach Brian Jacobs did a really short video that gives you a simple drill to improve your swing plane. I especially like it because it doesn't require training aids, which means you can use it on the course during a round if necessary. Here, take a look:
All you do is take your finish position -- that is, your "pose" after the ball is long gone -- and then, without changing your arms or shoulders, turn 180° so you mirror that position at the top of your backswing.
I know it sounds ridiculously simple, but don't underestimate this simple drill. If your "top of backswing" position mirrors your finish position, and if you make sure they match when you make your swing from backswing to finish, you're on plane. It just doesn't get any simpler than that.
And you can use this drill anywhere you can swing a club, which means you can do it in a few spare minutes every day in your backyard. Just do the drill a few times -- pose your finish, mirror it to the backswing, then swing back to the finish -- then see if you can swing to that top of backswing position from your address position. Once you can do that slowly, gradually speed it up to your normal speed.
If your swing plane is off, this drill will help you. Just add it to your practice routine and see.