The inimitable Miss Snark had a lot to say about the query process. Reading the archives of her blog can be a real eye-opener. One point she repeatedly made is to research agents before querying--don't just send out random queries to people you know nothing about. In her words:
"If you are querying a novel, you get an agent's name, contact info, and you learn about them by doing ALL of these: 1. Read the agency's website. 2. Read the listing on AgentQuery.com, Preditors and Editors, and Writers Digest. 3. Google the agent's name to find interviews, writers conference listings and other misc info. When you have done all three, you "know about" an agent and you may query."
Personally, I'd add two more useful places to get info about agents and publishers:
Absolute Write Water Cooler Bewares and Background Checks:
And Publishers Marketplace (costs $20 per month, but you can just join for one month and learn about all the agents and editors on your wish list and then unsubscribe):
Do you have any favorite ways to learn about publishing industry professionals?
Good luck in your search.