i work several clubs near where i live. the best thing to do is contact the club promoter for the night you want to work. tell the promoter that you'll pay either a upfront fee, or a percentage of what you make. also offer the promoter a disc of all the photos you've taken for the night. that will help sweeten the deal.
for instance, i work a club every saturday night. since im there on saturdays, i told the promoter that i will help promote the club by advertising on facebook/myspace. i also give him 15% of my profit. some of the pics i take i post on facebook, and have people tag themselves in the photos. so not only is he getting free advertising from me, he also uses me to promote and take photos at other parties during the rest of the week.
will you make tons of money doing this? you might, but dont hold your breath. picture this as a way to make some extra lunch money while you are having fun