If you Google "query advice", there's already a lot of great tips out there. Let me save you the trouble. Here are the ones I found most helpful.
Query vs. Request; don't screw up your subject line. If your pages have been requested, make that clear.
You don't hear a reply or you get a no for a variety of reasons. Quit stressing. Keep querying.
Most replies will be something like this:
“Thank you for the opportunity to consider the opening pages of (NAME OF MANUSCRIPT).
Unfortunately, I did not connect with the submitted material enough to consider your project for representation.
This business is incredibly subjective and I have seen authors that I pass on go on to critical and commercial success. What might not work for me might be the perfect thing for the next agent on your list. I wish you the best of success with your current and future writing endeavors.”
When agents start telling you why they didn't like your story, you're either lucky and they felt generous, or they REALLY liked it but just couldn't... which means you're getting close, and you're doing a lot of things right.
Don't mope. If a rejection comes in, or the reply deadline passes, update your spreadsheet and send another one out.
You DO have a spreadsheet, right? Don't miss the opportunity to track who is in what agency... after you send 30-40 queries out, you start forgetting who's who.
Be professional, online, offline. Don't jump onto Twitter and gripe about how no one is getting back to you, or about how all your rejections have been impersonal. It's okay to commiserate with other writers about the process, but keep in mind that agents are reading your tweets, too. Maybe even the ones you queried. Don't give them a reason to take offence.
Keep writing. Do something else. Push the querying forward in the in-between moments: like the mornings before work, or the evening after dinner. Don't waste your precious lunch hour or working hours stressing about it.
There is SO much good advice out there already on query letters, so all I will say about it is, make it good, don't stress, query 6-8 at a time, and always feel free to tweak. I did a bunch of Twitter contests, and having to pitch a variety of ways reframed my query letter for the better.
Happy querying!