I've been learning a lot about this subject lately. Here are a few tips I like:
* Find an Agent Who is Interested in Your Genre--Get on Agentquery.com and find agents that represent books similar to yours. Or, find a book that you LOVE and see who the author thanked (their agent).
* Follow Agency Submission Requirements --every agency has one. If the agent requires email submissions within the text, don't send attachments. Also, don't send anything unless it is asked for.
* Finesse your Query letter--it can make or break you, so get it right. You have one paragraph to catch the agent's eye. Be sure to thank them for his or her time.
* Don't Harass the Agent for Quick Response--they have enough to deal with without getting an email from you two days after sending the MS. asking if they've read it yet.