when i started assisting i made a list of all the photographers work i liked via web/magazines +++

i then emailed and called each one, to see if i could go and see them and show them my book, and see if they had any work going.

lots had no work for me but let me go and see them, to get advice to get other work and some of these called back later in the year when work came in.

a good place to start looking is the aop
www.the-aop.org who have a job shop and also give advice to asistants.

You can also contact the agencies aswell as some photographers limit their contact through them and some agencies also are part of the selection process. My biggest tip is don't go through the whole " I really admire you" "I've also wanted to work for you" approach keep it professional and more about your abilities. There is also no harm in aiming big. Some Top Photographers have more than 1 assistant and with a bit of patience you can work your way up to being a 1st assistant which can really set you up to become a photographer in your own right. Don't think because they shoot big campains there's no chance. I wrote to a Top photographer which although didn't get me assisting her it led to another photographer on the agency and then onto what I really wanted to be doing. Don't harass but don't sit back, try and think of a unique way of approaching them.