A few years ago a restaurant used singing dead rats in their commercial. I found it on YouTube. Watch at your own risk.
They were ugly, deformed, and wore festive hats. I still remember my horror. I also still avoid the restaurant. Someone truly believed that using dead rats in a commercial for food was a good idea. They might have thought it was funny. They might have thought it was memorable. They probably weren't thinking very hard.
Query letters are like commercials. There are query letters we think are brilliant, funny, and sure to make the agents come flocking to us. In reality, the letter is just a dead rat in a fancy hat. I'm not an agent. I don't have an agent, but I would like to get one some day, so I've been working hard to make sure that once my manuscript is ready I won't be sending dead rats to the agents I'd like to work with.
How can we make sure we aren't sending out dead rats? I'm still working on figuring that out.
1- Submit your query for review on an online forum like Absolute Write. You will get a lot of conflicting suggestions, and occasionally a troll will try to hurt your feelings. Get thick skin.
2- Writeoncon is also coming up! WriteOnCon is a forum based writer's conference. Plus there are agents involved.
3- Submit your query to online contests. I'm taking part in a superintern contest right now. The intern hasn't commented on my query as of yet. I'm really hoping it isn't a dead rat. It's hard to know since I'm so close to it. http://motherwrite.blogspot.com/ will resume contests in September, I think.
4- Follow the submission guidelines. Most agents will stress what they like and dislike in a query. It's not too hard to find if you Google their name.
5- Follow agents through Twitter.
6- Read http://slushpilehell.tumblr.com/ and see some dead rats for yourself.
7- Go to writer's conferences and take query classes. I had the opportunity of having my dead rat shredded by Holly Root at a writer's conference last month. I learned that my dead rat was too fat and should lose the hat. (This is starting to sound like an early reader.)
The main point here is that you have to do some research and you have to let someone else see your work. If you want to work with an agent, don't send them a dead rat.