Working with an assistant is both a blessing and a curse. It is great because you have someone in charge of your gear, holding lighting, and most valuably, someone who is there for you. It also makes things more difficult because you end up directing two people instead of just your subject.
My assistant, John Bartholomew, is a fantastic assistant because he is passionate about photography, and is a big NFL type of guy who is a gentleman with good manners. This means that he can carry 60lbs of gear, hold lighting at a great hight, and can create small talk while I'm setting up and creating shots in my head. Most importantly , he's like a caddie. We've worked together long enough that he can tell what I'm going to do next and if I'm overlooking or forgetting a small detail.
In the beginning, you have to give your assistant direction BEFORE giving your subject direction to avoid your subject falling apart while you're telling your assistant what you want.
Posing and lighting:
In this image I had my assistant hold a flash on TTL at 90 degrees from me, and the subject's faces and shoulders towards the light. I then exposed for the SKY and the Flash automatically lights them perfectly. I also had them lean toward the camera to correct the lens perspective because I was laying on the ground!