Bert Stephani has posted recently a blog article "Be an Assistant". I had a great pleasure to be his assistant in his recent Diva Shoot, where he invited a bunch of women to his studio (called 'The Barn), and he shot portraits in two different sets. We met a week before the shoot to prepare the sets, which was also lots of fun. Although my assisting during the shoot was mainly about taking behind the scenes pictures, I have to say that even during such assisting, I learned a lot.
However, assisting is so much more than learning how to take pictures. While you learn, you also observe things that you might have already knew before, which is also a great thing to remember these for the future. Basically, the things you knew before get better embedded in your memory and the likelihood that you will forget them in your next photo shoot gets much smaller.
But assisting is so much more than that. You also learn how to talk to and approach a client during a photo shoot (especially because every client is different) and you can learn much more than the photo shoot while talking with a photographer after a shoot. Although I was just shooting behind the scenes pictures and I helped Bert with preparing the sets a week before, I learned a lot! Make sure to check Bert's website and portfolio at http://bertstephani.com.