In the light of my experiences I recomend the following method...
1. Write a really good manuscript.
2. Compile a list of agents using such sources as the writers and artists handbook.
3. Before submission call each agent to determine that they're open for submissions.
4. Make your submission following the guidelines given by the agents.
5. Try to make 5-10 submissions a week and keep submitting until every single eligible agent in the world has a copy.
6. Wait for the rejection letters to roll in.
7. If after six months you still don't have an agent return to step 1 and start all over again.
Good luck, I've done it so it can't be that hard.