I used to be the editor of a letters column in a magazine, so I have some experience with this.
First and foremost, BE BRIEF!!! Most of the letters that I got were WAY too long! If the newspaper publishes a guideline for letter length take that as an ABSOLUTE maximum! Shorter is ALWAYS better and will ALWAYS make your letter more likely to be printed. If the newspaper doesn't print a guideline I would suggest no more than 200 words.
I know, you're thinking, but I have a lot that I need to say! No you don't. I'm sorry, but if you try to say all that your letter will just go into the circular file and then all of your points get lost. Pick your one most important point, make it well, make it briefly, and then leave out everything else!
This is the single, most important point about getting letters published!
Second, keep it clean. You would be amazed at how many letters I got that included outright profanities or vaguely obscene expletives. These letters always went immediately into the trash. I suppose the writers used profanities because they wanted to make their points strongly. In the end they only prevented their points from being made at all.
Third, check and double check for proper spelling and grammar. If you don't then the editor has to fix those errors and, frankly, that's just too much effort. Instead the letter will get tossed. Worse yet, if the editor doesn't agree with your opinion he might just print your letter as it is and you come off sounding like an illiterate boob. Believe me, that is NOT going to help your cause!
The majority of letters that get rejected are NOT rejected because the paper is being partisan. They are rejected for the reasons above. I really don't wish to offend you, Miggy, but your letter contains grammatical errors, run on sentences, and is just too long. You make good points, but I wouldn't print your letter either.
Here is a suggested revision...
Surprising nobody, the Miami Herald blames the Virginia Tech Massacre on the NRA. The Miami Herald, among others, celebrated the concept of a Gun Free Zone as a "sensible method" to keep people safe. Unfortunately, the tragic event at Virginia Tech proved the concept deadly wrong.
The NRA warned that a Gun Free Zone would create a target-rich environment for any criminal or deranged individual. They were ignored by those intent on selling people a false sense of security.
The NRA and law abiding gun owners are not to blame for the Virginia Tech massacre. Instead, blame those who supported Gun Free Zones and assured us no one would violate these “safe” areas. Jose Varela, Arthur E. Teele, and Cho Seung-Hui proved otherwise.
That's about half as long as your letter, but makes the same point. Also, you put "sensible method" in quotes. Is this actually a direct quote from the Miami Herald? If it is not, it should not be in quotes.
Good luck to one and all working to get a letter to the editor in print.