Most unsolicited manuscripts end up in the slush pile. Publishing houses and agents recieve hundreds of these a week and usually only resort to the slush pile when desperate.
To increase the chances of your query letter being read, meet the editor or agent face to face first. How? By attending writing and book festivals where publishers and agents will be present. Take a quick moment to introduce yourself, pitch your idea, and trade business cards. When you send your query letter & synopsis, state in the letter than you met them at the festival and you are sending this per their request. (And make sure they did indeed request it - lying will get you nowhere!) Author Catherine Coulter suggests having your photo taken with the editor and including that as well. Anything to get them to spend a few extra minutes reviewing your submission!
The owner of Echelon Press says she will not work with an author she has not met face to face, so you can see how important it becomes to put yourself physically in front of these people!