Read the acknowledgment pages of several books you like, or (better yet) books that are similar to your own. The author will usually give an acknowledgement to his or her agent and will list the agent by name. If not, there are still ways to find out who an author's agent is (if nothing else, type the author's name into Google and then agent and see what you come up with).
Get a good list of at least five top agents who deal with the same type of books you write, or who have repped authors you like. Then try to get the most up-to-date contact info on those agents that is available (P&E is often a good place to start, but there are several online resources available). I would say start there, query those agents first, then work your way down, starting with:
1.Well-known agents who have repped works you like or are similar to yours
2. Well-known agents who have repped other works (not necessarily your top five, but that you still admire).
3. Well-known agents who are noted for repping the kind of books you write (regardless of who their individual clients might be).
4. Well-known agents who've posted a blog you've read and whom you feel, either because of past sales, reputation, or personality, might be a good match for you.
5. Newer, lesser-known agents who are actively seeking clients, and are linked with a reputable agency.
This is usually a good way to start and helps keep the process from becoming overwhelming.
