Advice during the publication process?

I'm supposed to start grad school next year but am taking a year off to work on my novel. I'm in my 3rd serious revision. Besides a few magazine articles, I have not been published. I know how the process works: query letters, agent, publisher. What I'm looking for is advice during the process, how to make a query letter look as good as possible. What can give me a leg up etc. Also, have any of you been through the agent or publication process? How many agents did you query before finding yours(if applicable). I would love to hear experiences, regardless of the outcome.

Added (1). *I'm on my iPhone so forgive any mistakes.

Query letters need to be to the point. No more than a page. Example:

Dear (agent's name),

I'm xyz and (1-2 sentences stating your intent).

(2-5 sentences explaining plot)

My novel, TITLE, is complete at xyz words. This is my first novel. (one sentence appreciating their time).

Thank you,

Your Lovely Name
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A previously published novel would give you a leg up. Being a first-timer won't kill your chances either. Make sure those 2-5 sentences where you explain the plot grabs the agent. Most query letters get rejected because the writer doesn't properly research what a query letter is. My uncle was an established author and guided me through the query writing process but he didn't teach me anything Google or Query Shark can't teach you. I'm not sure how many queries I sent. A good friend of mine kept all of his rejections in a file but not me. A guess would be in the 20s.

Treat your query as seriously as your novel. Write it, revise it, put it away, look at it after a week and revise it again.

I would suggest extensive research on everything you can learn. AbsoluteWrite is a great site for information. Also, you might be amazed at how many podcasts are out there as well as iTunes U. I'm still learning from podcasts like Odyssey Writing Workshop.