I spent six years working on Capitol Hill. One as a staff aide for a particular Senate committee, two as a legislative correspondent for a Senator, and three as a legislative aide for the same Senator.
For those of you who don't know, a legislative corespondent (LC) is basically a fancy title for a young twenty-something just out of college who receives the letters and emails that you write, and is usually the person who writes you back. Generally, Congressmen have one or two LC's who do all of their letters for every issue, while Senators have a whole team of LC's that handle letters for certain issues that they're assigned (IE: a military LC, a healthcare LC, an education LC, etc.). Here's a few things that I picked up as an LC that might help you should you choose to write your Congressional delegation:
* The "boss" will almost never read your letter directly unless they know you personally. But he/she will likely ask their LC's if they've been getting any mail on certain issues. My boss used to ask if we did almost daily whenever there was a hot issue being discussed on the Hill, and would want to know what percentage of it was for or against the issue. Believe me, if a majority of the letters they are receiving are against gun control legislation, they will hear about it.
* Don't send form letters. Form letters are letters that special interest groups will send in on your behalf. You'll usually get them in an email and all you have to do is type in your name and hit 'send.' They all look identical, and the staff knows that you put absolutely zero thought into the issue, and therefore will assume that you probably don't give that big of a shit. When the boss asks if he's getting any mail on a certain issue, he often asks how much of it is form letters, and how much of it is real letters. Real letters (while rarely read by the member of Congress) are still a lot more powerful in the grand scheme of things.
* Don't use snail mail. Everything that gets sent into Capitol Hill has to go through the Capitol Police and is scanned for bombs and anthrax. It usually takes weeks for it to get to its destination, and more often than not, the issue is long over by the time it gets there.
* Don't be a dick. The kid who reads your letter makes about $25K a year and lives in one of the most expensive cities in the country. He spends all day getting yelled out by assholes, and he's just as likely to toss your letter to the side and forget about it for a few weeks as he is to respond to it. Be professional, but make your concerns heard.
* Keep it short and sweet. Remember, the Senator/Congressman will most likely never see your letter, and the guy who will see it spends all day reading long, drawn out sob stories that basically all say the same thing. He doesn't need your life story. A few sentences will do just fine. Let him know you care enough to write your own letter, but keep it short enough to not bore him to death.
Keep in mind, this is what MY old office did. Other offices do things differently, and some offices couldn't care less about mail one way or the other (Boxer and Feinstein were notorious for not giving two shits about their mail or phone calls). If you do have a few mintues to spare, however, take the time to write in (especially if your member is a Democrat). It can, and often does make a difference.
Anyways, I hope this helps.