Advice from the fabulously helpful Dystel & Goderich Literary Management blog:

The perfect query letter does not exist. (Well, perhaps it lives in the fantasy realm of unicorns and dragons, but certainly not in our day-to-day publishing world.) And, yet, everyone seems to be chasing the formula for that elusive, perfect query letter (EPQL) and its pursuit is giving a lot of people agita and heartburn. It's a recurring theme during the Q&A portion of agent presentations at writers conferences. Many internet sites and print publications aimed at writers spend a lot of time on the subject and, in talking with individual authors, it seems that confusion about this subject is universal.

So, I will try to elucidate what makes a query effective -- not perfect, mind you, just effective -- for us here at DGLM:

Read on!