Recently I read this specific advice on writing a cover letter reiterated in different forms many times:
Address your cover letter directly to the hiring manager or recruiter.
Nothing says "I don't care about your company" like an opening of "To Whom It May Concern." That may have been OK before the advent of modern technology, but today it generally takes as little as a Google search or a phone call to figure out the name of the hiring manager. Addressing your letter to the correct person (and spelling their name correctly!) will automatically ingratiate you to the reader and show that you've spent some time researching the company and position.
But what if you turn Google inside out and could not find the name of the hiring manager? Should you respect the decision of the company not to disclose this information, or should you make a phone call?
My opinion is that you shouldn't. Doing so just interrupts somebody from his work and wastes his time all in order to artificially boost yourself to the front seat. Not to mention that if many candidates have the same attitude, it can be quite annoying.