Writing a query letter can be the most mind numbing task of submission. You certainly don't want to spend all that time on your MS, if your query letter isn't up to par.
These are helpful links about do's and don'ts of querying....For specific guidelines, be sure to do your research on the agent/ editor you are querying.
http://evileditor.blogspot.com/p/frequently-asked-questions.html What will you find here? A blog based on why you don't get published. Created by an editor, who takes query submissions and picks them apart...talk about the ultimate critique! While the purpose of submissions is for entertainment value only, I can't help but feel there would be some kind of major silver lining here. I have spent time reading this blog, browsing the archives and searching for the most relevant issues with querying that I'm experiencing. I haven't decided whether or not to submit my query to this blog for a smack down, but I'm heavily considering it!
http://misssnark.blogspot.com/search/label/crapometer-cover%20letters What will you find here? A CLOSED blog- that's right. This blog is no longer open...what a shame. The good news is, the information is still available on this blog. Here again, an editor took query submissions and explained why they didn't work. The archives are chalked full of examples. Worth the read.
http://queryshark.blogspot.com/ What will you find here? An active blog for submitting queries to and receiving feedback. Take care to read the guidlines for submitting your query and read the archives. There is a lot of helpful info, so take time out of your day to read other submissions.