1. To drive safely; to stay on the road. (Usually said imperatively.) Whoa there, son. I know you're only just learning, but try to keep it between the ditches!

2. To behave properly or appropriately; to stay out of trouble or harm's way; keep to the straight and narrow. (Often said imperatively.) Okay, Bob, I'll see you after you're back from your trip. Keep it between the ditches now, you hear?