This article is from the misc.writing Writing FAQ, by Wendy Chatley Green firstname.lastname@example.org with numerous contributions by others.
The easiest method: Sell your book to a publisher.
Then write letters to agents, asking them if they'd like to
earn their 15%.
A somewhat harder method: Send the book
over-the-transom to agents who are looking for new clients.
A colossal waste of money: Pay somebody an up-front
reading fee. There have been a very few exceptions, but
99.9 per cent of all decent agents don't charge up-front
reading fees; they make their money by *selling* your book,
not by reading it.
[The times, they are a-changing. As the function
of slushpile weeding is shifting from publisher to agent,
many agents see reading fees as the only way to recoup their
costs. It is still true that you should try to find an
agent who doesn't charge a fee first, and that you should
check the credentials of fee-demanding agents very carefully
-- make sure that their major source of funds is selling
writers, not reading manuscripts.]