I left a word out, which I hate, because you should read Josh Olson's recent piece in the Village Voice:
You are not owed a read from a professional, even if you think you have an in, and even if you think it's not a huge imposition. It's not your choice to make. This needs to be clear--when you ask a professional for their take on your material, you're not just asking them to take an hour or two out of their life, you're asking them to give you--gratis--the acquired knowledge, insight, and skill of years of work. It is no different than asking your friend the house painter to paint your living room during his off hours.
Or, my favorite lines: "It rarely takes more than a page to recognize that you're in the presence of someone who can write, but it only takes a sentence to know you're dealing with someone who can't. (By the way, here's a simple way to find out if you're a writer. If you disagree with that statement, you're not a writer. Because, you see, writers are also readers.)"