One of the great things about a site like this is, you get a chance to read as many query letters as an agent.
When you have read and critiqued so many queries (I'm thinking 100+) that you feel a bit ill when you read the first sentence of someone's query - then you're ready to write your own. Your head will be in the same place as the agent you're writing to.
Good luck!
It's funny, and it's true.