I came over the top for many years. Here's what finally fixed it:

* Closed stance drill (I hit balls with my RIGHT foot placed about 10" behind, or else my right foot directly behind my left foot), to encourage an inside swing path.

* LAG/Pump drill. From halfway back, set your arms/wrists in the correct swingplane, and wrist cocked (lagging) position. Allow your arms to fall, releasing the clubhead through impact. It felt as if my wrists were going to impact my right hip, but it doesn't.

* More inside takeaway. I slightly changed my takeaway more to the inside, which enforced keeping the shaft/club on that plane on the way down.

* SpeedStik. I've reviewed this elsewhere, but I'm very convinced that this device helps your tempo, and keeping the club on plane. It's hard to swing it fast, unless you're on plane properly.