I just go around and around, turning 90 degrees on square corners. My brush hog seems to discharge more to the left, so I make sure it discharges to the side thats already cut, so I go clockwise.
When it starts to get small enough that its hard to turn sharp enough, I split the remaining rectangle in half and go down the edge, turn and take a swath out of the middle, then down the edge, etc. This way, you don't end up with a little skinney section that you have to back up to turn each time.
I have some oddball shapes in my place, plus a lot of trees I have to go around, but this the way I do it on rectangles.