Come recommended. The first thing an agent wants to see is that you are someone who has already verified. Comb your resources for people who will vouch for your talent and ability. Ask well-known industry professionals and former teachers for their recommendations, then add that front and center in your cover letter. This tells the agent that you are desirable to others and are pre-approved.
List all of your selling points in the second paragraph. This is the meat and potatoes part of the letter, where you concisely list all of your selling points. Include key recent acting experience, commercials you’ve done, schooling, award winning productions, accolades and positive reviews you’ve received. These attributes should be written with originality and panache, without being overly wordy.
Ask for what you want in the third paragraph. Tell the agent you’re seeking representation and would like a meeting at their convenience. This is also where you can list your website address where the agent can see your demo reel. You can also offer to mail them a demo reel on DVD. It’s much easier for someone interested in seeing your work to look online rather than wait for a mailing. If you don’t have a website, it's time to get one.
Wrap things up with a sincere thank you for reading your cover letter and resume. Let them know you’ll look forward to hearing from them. Be positive, but not overly cocky. You have to come across and interesting and someone they'll want to meet and work with. Don't come across as needy or overbearing.