Here are some good basic guidelines for agent query letters (mostly as they apply to books) from a column on writersdigest.com. Click here to read the full article.
When Writing a Query Letter Do…
- Address the agent by name
- Cut right to the chase
- Sell your manuscript
- Explain why you’ve chosen to query this specific agent
- Mention your platform (if you have one)
- Study other successful query letters
When Writing a Query Letter Don’t…
- Be arrogant
- Include your age
- Tell agents that you value their time
- Include writing credits that aren’t meaningful
GREG MILLER - Writer & Writing Coach 
These are very sensible guidelines. But I wish someone would write a set of guidelines for the agents to use when (if only!) they replied.
Addressing the writer by name and giving some indication that they have actually read the manuscript would be minimum requirements along with a well-argued reason why they are rejecting it, which, in my experience, they inevitably will do.
Having sent many query letters along with the 50 pages of manuscript, synopsis and biog they often request, not to mention an SAE, and not even receiving the courtesy of a reply, perhaps I am jaundiced on this issue.
Agreed, Allie. It sounds like time for some self e-publishing for you!