I'll share some experience I have when working for
Jing Quek last year as a Assistant/Intern.
Well, as a assistant, you are basically the guy who helps him get his shoot done. That means answering calls, emails talking with clients, chasing for money, buying coffee, lunch etc. Its a whole host of things that you have to do to help the biz running and you would require quite a wide range of skills in order to accomplish them.
One of the main tasks that I had to do was getting ready all the equipment for shoots. That would mean packing the gears, preparing the cameras, lenses etc. Photographer will just tell you I need x number of lights, reflectors etc and you go figure out the rest. Pack them well in advance and make sure you don't slip up. Photographer will not be happy if you slip up on such things. Also, be on time, or even better, early. NEVER and I say again.. NEVER ever be late. Be prepared to be fired once you keep falling behind the schedule. Also, anticipate if he might have forgotten something. Like, wouldn't you need this reflector, or do you need the extra pocket wiz, etc. No harm being safe. Also, learn how to set up equipment before the shoot. The photographer is going to be really distracted if he has to teach you on the spot. And worst still, you keep forgetting.
At the shoot, be fast and careful with the equipment. I was working with Profoto lights and they are way into the thousands and 10 thousands.. so be careful when setting up, but yet remain really quick. Time is money, and wasting time only pisses the photographer. If possible, ask the P-graher, what kind of setup he wants. What formation etc. This will help aid your time when setting up and stop asking him silly qns like.. err.. Is this suppose to be here etc.
During the shoot, keep a look out for anything that might go wrong, lighting not firing etc. PW out of battery etc. Tell the photographer. Run if you have to make changes. Who likes waiting isn't it?
Once the shoot is done, be quick in packing. Again, be careful not to leave anything behind. Get accustom to a particular workflow so that you know how things function and run.
Back at the office, answer calls, help with planning, scheduling, emails etc. Learn to go hungry if you have to when schedule is tight. Also, learn to be prepared to move out on short notice, e.g. eating your lunch before turning up etc.
A good assistant can make the life of a photographer alot easier. Best of course if you drive. That will save the photographer alot of time thinking and driving you around.
Lastly, the biggest myth of a assistant is that he needs to know how to shoot well. thats one of the biggest myth that I never understand why. Good knowledge helps but its not necessary. More impt is you learn how to operate the equipment as you will be seldom-almost never called upon to shoot. If you do your job well, the photographer will definitely treat you well.
Have fun and Learn! It was one of my best times in my life working for Jing and learning from him. =)