Firstly, I think it's important to look at querying as a process, potentially, a lengthy, ongoing one.
It's also important to work at building your knowledge and developing and refining your skills in this area, just like you would in any other.
I can tell you from personal experience, querying can be a disappointing and frustrating experience, particularly when you've worked hard on your project, and at researching your agent list and improving your letter.
But it's important not to give up and to continue to be open to new information and ways of working.
One tip I received recently, was, in addition to including the word "Query" in the email Subject line, to also state the project's title and genre. For example, the subject line in my email query read: "Query: Commercial-Literary; Title: SEVENTEEN SUMMERS."
Doing this ensures an agent/editor knows what they are about to read, and might help them determine when they want to read a query in that genre, that is, when they are most receptive to it. As I see it, this can only help your cause.
Below is the most recent query letter I used; I hope you find it useful.
Please consider reading my 72, 000 word, coming-of-age, commercial-literary novel, SEVENTEEN SUMMERS, a “displaced memoir” of hope and love, overcoming violence and hurt.
Teenager JAMES CAIN is struggling to breathe amidst the crush of a dysfunctional family and a prejudiced peer group with a herd mentality. But when he trips for a spunky girl of mixed race named SARA LINDS, he’s no idea the wild ride their intercultural relationship will unleash. Thrusting them on a passionate and confrontation filled journey of discovery, and understanding of themselves, one another, and the invading world.
Thank you for your time and consideration.