If the vast majority of letters on a particular topic seem to all take the same position, your letter will stand out and have a greater chance of being printed if it takes the opposing viewpoint.
Don't ask editors to print your letter anonymously. Most newspapers won't, so you'll be wasting your time. And it defeats the purpose of free publicity.
Don't write a letter for the sole purpose of publicly humiliating a company you do not like. These are seldom printed. If you have a consumer gripe about a particular company, call a reporter and suggest it as a story idea.
Don't write a letter that exceeds the length limit or you will make editors spend valuable time cutting it. The "Letters to the Editor" page will usually include guidelines for writers. Follow them.
To learn more on how to and how not to get noticed when writing letters to the editor see Special Report #4 "How to Write Crisp, Compelling Letters to the Editor to Promote Your Product, Service or Favorite Issue"