Since many of my writer friends are at the agent querying stage I wanted to share some tips that helped me not only land an agent, but find the right one. The first and most important thing you must do it make sure your work is the very best it can be. Don't submit after writing only one draft. My rule of thumb is four edits. I do a mini-edit right away because I hand write everything first then do a little edit as I put it into the computer. Then when the entire manuscript is done I do a read through edit on the computer. After that I print it out and take a red pen to it. I put all my edits in the computer, then do another read through edit. After my workshop in Tulsa I've decided I'll be adding a fifth edit where I read it aloud to myself (you'll be amazed at what you catch!).
Once the editing is finished you need to decide who you're going to submit to. Treat this as carefully as you would the selection of a job you plan to retire from. You need to be able to work with your agent well on all levels. Look at how they communicate. Are they on Twitter, Facebook, or Myspace? Do they blog? If they don't have a strong web presence chances are they won't keep in contact by e-mailing you. If that's fine by you then they might be a good fit. However, if you're like me and practically live online, you want someone else that does too.
DON'T mass query. By that I mean, don't mail the same query to several agents at one time, within the same e-mail with a general 'to whom it may concern' or 'dear agent' opening. That will get you auto rejection from them 9 1/2 out of 10. They're people and they want to be treated as people and individuals. Research them and make the query letter personal to a degree so they know you're submitting to them specifically for a reason. Check around agent websites. Some post successful query letters. Agent Kristin Nelson does this. http://pubrants.blogspot.com/
If you've researched the agent you're submitting to the next part will be easy. Make sure you follow their submission guidelines! If you don't many of them will auto reject you. They have far too many queries to worry about the people who can't follow guidelines.
Probably the most important piece of advice I can give you is, learn to take rejection in stride. Don't get mad, don't demand to know why, don't ever get snippy with them (all of New York will know if you do. Agents talk to each other!), and don't get discouraged. These are tough times and there are a lot of good writers out there trying to find an agent.
Another great piece of advice I picked up from NY Times bestselling thriller writer James Rollins is this: Send out ten queries at a time. When you get one rejection, send out another query to another agent. Do this every time you get a rejection. That way you'll always have ten prospects out at a time. This helps keep your spirits up. I'd add to that, if you get a request from more than one agent for a partial or your full manuscript, let them know who else is looking at the partial or full. It doesn't really matter if other queries are out, only if other partials or fulls are out. It's common courtesy because now they're really considering you and giving you their time. Be considerate of them and the relationship will get off to a much better start.
Keep writing while you wait to hear back, it will help keep you sane. Check out these articles by Writer's Digest on submitting: http://writersdigest.com/article/10-submission-tips-for-querying-an-agent/
and the basics of a query letter: http://writersdigest.com/article/basics-of-a-solid-3-paragraph-query/