Hot tip: The "acquisitions editor" whose name you found in Writer's Market doesn't exist. She ain't real. Think you're coming off professional by addressing your submission to the proper editor—the editor you've never corresponded with, spoken to, or even found mention of on google? Put down your crack pipe: there's no such donkey as "Ethel Largetits, Acquisitions Editor" (or "Barry Bigschlong" or "Annette Sexton"). Publishers frequently make up a fake "acquisitions editor" so they can tell which submissions are solicited or come from informed sources and which are coming from people who've found them in Writer's Market. When a manuscript comes in addressed to Ms. Largetits, it's an instant tip-off that whoever sent it is not in the know. Bing! Into the slush pile, where your intrepid unpaid intern (aka "Ethel Largetits") will skim it, send you a signed rejection letter ("sincerely, Ethel Largetits"), and forget you exist.
If you really want to come across professional (and not as a first-class SUCKAH), do your homework—put down Writer's Market, hit the internet/phone/book fair/real world, and make sure you send your manuscript to a real person. If you have an agent, she should know the real names and contact information of the people you want to see your manuscript. Sometimes we get submissions from so-called "literary agencies" addressed to Ethel Largetits and I just have to cry for the poor sap trusting these jackasses to represent their work.
Disclaimer: Sometimes, the acquisitions editor listed in WM is real. But not always! So watch your fucking back!
Summary: Get wise! Figure it out! And don't trust nobody...except THE INTERN.