There are so many people writing these days and so few major publishers. Most large publishers use literary agents as a means of screening manuscripts; getting rid of the awful ones up front. Publishers who don't charge a fee to publish works, screen heavily to find the best bets before backing the project with company funds.
Writers use blogs, twitter, and facebook to get noticed. Writing samples are posted their websites and writing forums. Here is a new one: http://thenextweb.com/media/2011/10/03/pubslush-launches-a-kickstarter-platform-for-book-publishing. If you read the article, you know as much as I know about it. I'm not endorsing it one way or the other; just tossing out this bit of information for you to check out.
I've seen bulk queries come through that obviously are a shotgun attempt born out of desperation to get past the first hurtle and have their full ms requested. To date, every query letter I've received that began, "Dear Sir" hasn't made it past me to the vetting committee.
One committee person asked to read my rejects before I wrote the rejection email, then sent a note that they agreed that the work was so substandard that it wasn't worth taking it to the full committee.
It is extremely important to put your best effort forward every time. You only get one shot at a first impressions and the competition is stiff.
Try not to query when you're tired, feeling low from a rejection letter, or hurried. It will make a difference if you're rested and feeling confident about your work.
If you've self published, you know that your book gets turned away from some sites and some reviewers. It isn't a level playing field. Take a look at this place: http://filedby.com/author/nadine_laman/2238025.
No matter what, write your best work and believe in yourself and your ms.