*Always address the letter to a SPECIFIC agent, not an agency, also not a "Dear Agent" you want "Dear X".
*Make the letter personal but professional. Does this agent represent the genre of your book? Did you like the book? Why do you feel like your writing would be a good fit?
*What is the audience you're aiming for? YA? Adult? What genre is your novel? Paranormal? Romance? Heavy fantasy? Make sure to include this information SOMEWHERE in the letter.
*What is your novel about? --This is the hardest part of the query letter. Personally, I'm usually too emotionally involved in my writing to take a step back and say "This is my plot". I get help from friends who read my material and say "This is what I got out of the story". Make it interesting, use proper grammar, use correct spelling. It's like a mini-story.
*Do you have credentials? Have you been published before? Where? For what? Would the agent have heard of you? Do you attend conferences?
*Try to keep this under a page (approximately 250 words)
*Sign your name and give information on how to contact you (name, address, phone number, e-mail address, etc)
*Lastly, before you send that letter, make sure you've checked the guidelines. Did the agency ask for pages pasted into the body of the e-mail? A synopsis?
Those are the fast easy tips that I've come across in a lot of industry blogs. Hope this helps! Good luck! Oh--and if you get rejected, don't worry. It happens to all of us. Just keep trying!