You don't say which way you're going to and from.
My tendency is to swing out to in, and the most effective drill I've been given to work on (for me) is to push an alignment stick into the ground at the same angle as your club shaft address and a couple of feet to the right of your right foot.
You then take the club away outside the stick, and bring it back down inside.
I was told to work on it as a slow motion drill, but I got cocky and started making full swings. Smashed a ferrule on the end of the stick, so now I only do the slow version.
