There are a few ways to pick the clients you want to market yourself to as a photographer.
www.lebook.com
You can not only buy a page and show your past campaigns on Le Creative, linking yourself to buyers, but.... If you don't want to throw down the $$$ to promote yourself on Le Creative you can use the site to find out what AB's, AD's, and CD's are working at what agencies, and on what accounts. Knowing who you're talking to, and the work they've done is important.
www.pdnonline.com
I find that it's always good to check out the "People on the move". It's hard to find the right people to talk to when you are making those calls to promote your work. Sometimes it's not on ADBASE or in Lebook. Try looking here. Also "Who's shooting what" will give you some insight into who is working on what, and what photographers they have used.
www.mediabistro.com
I check out the accounts in review. You can see what accounts the agencies have just won. This is a good time to send your books if they're appropriate for that bright shiny new account.
It's good to keep in mind when dealing with ad agencies that the Art Buyer can love your book, but it still has to get approval from many others, most importantly, the client they are trying to please.
I'll keep adding, but it's Friday night, and I have to stop geeking out.