Also, many literary agencies have writer's guidelines available for the asking. Write a strong, professional query letter. In one or two typed single-spaced pages, describe your book. If you have relevant expertise or experience, mention it. Include copies of any significant published writing samples.
Clips are fine.
Agents are swamped. Even the smallest agencies generally receive twenty or more query letters a day. Expect to wait a couple of months after sending a query letter. If you don't hear by then, you can call with a gentle reminder or follow up with a note. Send precisely what the agent requests.