a good assistant has to be able to interpret the photographer. this usually happens after working with the photographer on a few occasions. similar to dating a new girl

. you get to know them, through good times, and the not so good(like another assistant forgot to pack the other case of strobe heads, oops

). I've found that first of all, you have to be willing to learn. If you come into a job, acting all arrogant and "I know it all", as I did, really dumb idea on MY part

. Well, let's just say I learned my lesson after not getting called back by that photographer.
Anticipation is key! After working with someone for a while, and learning how he/she likes to work (even which side of their body they like to be handed things from in one case :confused:, but you pick up on these things quickly if you're alert.
Having camera knowledge is good, but if you're hired as an assistant, and you're told to man grip and lighting for the duration of the shoot, then its not your job to handle the cameras. Do only what you're told, and don't guess; its better to ask a dumb question than to make a dumb mistake (speaking from personal experience

).
look to have fun and learn. the point of being an assistant IMO is to be an "apprentice", similar to that of a carpenter or a plumber. You work your way up the ladder so to speak, and eventually you are put in charge of more responsibilities.
And I've always found that standing around, even if there's NOTHING you can do that is productive on set, ALWAYS look busy. The photographer might not notice that there is nothing to do, but they will notice that you are "working your butt off" all the time, and they will appreciate that.
Also, bringing them a bottle of water if you go to the fridge/cooler to get one for yourself(at least ask them first if they want one as well), then quickly go to get i, not walk

.
ask as many questions as you can without it getting in the way of the photographer or his 1st. they are there to focus on getting what the client wants, and you are there to do what they tell you.
just my 2 cents
-dan