What's going well, what's annoying you? How are you feeling? How's life? What can I do to make your job easier? Are you heading in the direction you want for your career?
Showing and taking genuine interest in your team is the mark of a true leader and will help you retain A players. Don't wait for any issues to grow and fester - get your team to talk about them so you can resolve them.
Weekly is maybe too often, every other seemed to work best for me.