The downside of the published market is that it typically draws hundreds of applicants for one position alone. So, should you even bother applying for jobs in the published market?  Of course; but spend only 15% of your precious job search time responding to ads - and the remaining 85% networking. To stand out from the competition, use these strategies when writing cover letters for both identified and blind ads.     
 
General Rules for Cover Letters
* Letters should be warm and personal. Avoid stilted, stereotyped phrases and job-hunting jargon like "utilize" instead of "use," "proven track record," etc.
* Send letters by email and US Mail, since emails can be intercepted by spam filters.  Email resumes and cover letters as attachments entitled:  "yourname.resume" and "yourname.coverletter."  In the body of the hard copy letter write: "I am also sending a copy by email for your convenience." In the body of the email write: "This is a duplicate of a hard-copy sent by US mail."  Email gives you speed; US Mail gives you certainty.   
* Print hard copy letters on a good quality laser printer, (600 DPI minimum), using plain 20+ lb paper.  
* Keep your letters brief unless there's reason for detail.
* Maintain copies of all correspondence for follow-up.
* Include your contact information on all correspondence: name, phone number(s) mailing address, and email address. 
 
Replying To Ads Where the Advertiser is Identified
Rather than writing to the person specified in the ad, find out who the hiring decision maker is for the position, and send your letter directly to that person. Research the company website to identify the hiring person; or see if someone in your network may know who this person is and can introduce you. Or, simply call the company and ask!  When writing your cover letter:
 
1. Explain your reason for writing to this individual. Do not mention the ad, or your materials will almost always be referred to HR, where your competitive position will be weak. Focus on your research and knowledge of the company. Decide whether an additional letter to HR would be helpful in your situation--or if it would interfere with your direct line to the decision-maker.