I stop by every Friday and inspect the work, then pay them for that week's work, and talk about the next week's work. Takes maybe 15-20 minutes. If I don't like how it's been done, I ask them to re-do. It also helps to point out how you want the little things done right before they start the next week's work. It also helps to give them detailed pictures of how you want things to look. It's nice to be able to walk into the office and bring up the pictures on the computer and show them. After a while they understand your expectations. No surprises, no runaway bill at the end. Be involved as much as you can without getting in the way.