I'm of the opinion that a good cover letter and cv addressed to the right person in a company can make a difference - find out the manager of the department you think would be your best bet, and address the cover letter to them personally. By targeting the person who is in the position to say if they could use you or not, you've a better chance than going through HR. Linkedin is a great way to find people who work in a company, if the company is large enough Linkedin will have a profile for that company, and will list the job titles of everyone who has that company as their current employer. Knowing a job titles is not enough on its own, though it is an improvement, but without a premium account you can't click on the job title and view the persons profile and get their name.

Here is the tip - copy the job title that sounds closest to where you want to be, "Marketing manager at xyzcorp" into Google, your first hit is likely to be the Linkedin profile page of the person with your target job description! Now you have a name, a job title and can confirm their location (important so that you target the right office), and you are all set to send them off some nice paper.