Also, I agree with Vijaya not to rule out subbing directly to publishers. GOODNIGHT, GOODNIGHT, CONSTRUCTION SITE was subbed without an agent and it's been on the bestseller's list for a zillion weeks! I believe SOPHIE'S SQUASH was a slush pile find, as well.
(But let me just note that if you sub to a large number of publishers and it gets rejected, it will be harder to find an agent for that particular manuscript.)