If at all possible, I find out the name and title of a live human being to address the letter to. If I can’t do that, I use “Dear Madam or Sir.” If you don’t like that, my three suggestions would be:
– Use a general title, like “Dear Customer Service Representative;”
– Address the letter to the company, like “Dear General Motors;”
– Do away with the salutation altogether by using a memo format instead of a letter format: “To: General Motors/ From: Mr. Scroph.”
But personally I find all of those to be more stilted than “Dear Madam or Sir,” so that’s what I continue to use (but only after I’ve made an effort to find a live human being to address, and haven’t been able to). Other salutations like “Greetings!” strike me as particularly tinny, though obviously YMMV.
And I always use “Madam or Sir,” not “Sir” or “Sirs.” Chances are about even that your letter will land in the hands of a woman, and she may not agree with the assertion that the masculine forms of address include the feminine. I’m a woman, and I don’t buy it myself. It’s not like it’s that much harder to use a generalized salutation that correctly acknowledges that the recipient might be of either gender.