I don't know if this is printed anywhere but this is something I have done with my wb who wanders all over the place.
When im on a track and he doesnt want to go straight I focus my attention on where I want to go and gently push him back each time he wanders. If he wanders to the right, I gently push him back with my right boot and 'wiggle' the right rein on his neck and the opposite for if he wanders right. As soon as he is back in the middle of the track I leave him be. Sometimes this goes on for a while but I have found my fella learnt pretty quick what was expected of him and now he is pretty good. We still have or moments but I think its more because he thinks everything is a big bad monster and its going to jump out and eat him if he doesnt steer clear of it :) we're getting there though