FWIW, I have worked in a Congress critter's DC office, albeit before 9/11. It was true that letters were more highly regarded than phone calls and emails and the pre printed stuff by various interest groups was kinda at the bottom of the heap.
9/11 has changed the procedure due to the anthrax attacks via mail and thus all congressional mail by constituents goes to a facility to be bombarded with radiation and scanned to kill off any biologics. This slows the receipt of the mail quite a bit (at least it was a few years ago). So, this is simply my observations from working in politics and dealing with politicians off and on over the years.
A) the folks that get the highest attention (aside from donors who have a different relationship with candidates) are those that do regular party work and not surprisingly these folks are more likely to actually get a word or two with candidates from time to time. Local political party organizations are remarkably open to someone that does hard grunt work and keeps their mouth shut especially if they are a volunteer. At party events, you often then get access to candidates and can give information to them in a less adversarial manner and they do not necessarily regard you as a gotcha type recording the whole thing for a sting.
B) Personal contact options, if you are Average Joe or Josephine citizen who does not appear like a crank or act like one. Town halls, campaign rallies, etc. can often give someone a very short window to access the candidate directly via questions or information--do not necessarily expect them to give public commitments right then and now but your information does seep through. Another good approach is most congress critters have local offices that are less busy than the DC office, particularly during recess in Congress. Go there and bend a staffer's ear but do not come across as a crank or threatening. The local offices do funnel information back to the DC office.
DC offices visits can also be helpful but remember that they are very busy, including staffers, during sessions and they can vary in helpfulness and have their own hierarchy. The Administrative Assistant is a chief of staff for the office, then you have legislative assistants that focus on the congress critter's legislative interests, depending on the position of the congress critter in seniority and committee assignments, there might also be another staffer or two from committee staff that are more or less assigned to do the member's work for that committee. Then there are secretaries and interns but you can still give information to them as well. Leadership offices in Congress have their own separate suites and are harder to access for Average Joe or Josephine because of restrictions in walking around Congress--but their regular office for their constituency is perfectly accessible in the congressional office buildings. Senators have bigger offices and greater staff so are generally harder to access--be content to deal with staffers. Be aware that there is a wide variance in how these offices treat visitors but be polite, be focused, and keep the message short. Occasionally, during travel, especially via DC and back, you might actually see your Congress critter and be nice, be polite, get your views across in a very short manner, and then leave them alone.
Next, there is indirect communication. The various pre printed stuff and the goofy mailer petitions etc. are not that useful. Congress critters and staffers are well aware that interest groups raise money through these mailers either directly or indirectly. The key thing for mass mailers, whether for commercial or non-profits or political, is to get you to OPEN THE ENVELOPE. Then you are much likely to give money of which a fair amount goes to the mass mailing firm. Thus, this material is designed to go to your fears, your greed, etc. rather than appeal to your rational side. Think of most political mailers/postcards that either say the other person is a bum, and/ or I will cause you to get more money/employment/happiness, or identity politics where the person claims that I am one of you and will fight to get your view through to (fill in the blanks--county commission, state legislature, Congress, and so on). Interest groups will give you mailers or phone numbers to Congress, etc. for you to GIVE THEM A PIECE OF YOUR MIND etc. and by the way can you give for the fight brother since it is so critical at this time. If you think of the tactics of con-men, there is a close resemblance to these in the marketing operations of interest groups.
So enough of what doesn't work well, then what does. A simple letter, do not worry so much about grammar, etc. as long as it is personal and does not parrot the talking points on TV vomited forth by "stategists" or your favorite pundit. If you come across as honest, concerned, and worried about the state of the country given a particular policy and you are well informed about that policy, then you might actually trigger a reaction. If time is of the essence, then fax the doggone thing--I have not dealt with the issue recently so it might also be possible to send it fedex or ups without going through quarantine but mebbe not. The shocking incident where a citizen actually spends their own money and time providing information on a subject might just cause some consideration of your points relayed to the Congress critter. You can also deliver such a letter in person to the local office for them to send and it is almost as good. One to two pages would be the max. If you can do a postcard, then do that but vague pronouncement like support the 2A ignores the reality that Congress votes on specific legislation and not principles. Congress is just capable of passing unanimously a resolution to support the 2A while also passing a bill that eviscerates it in actual effect. It is usually better to focus on whatever legislation is currently before Congress than vague new policy proposals.
Now we get to emails. Since 9/11 and technology spread, emails are probably the most common form but staffers also recognize that a lot of folks will simply vent. They obviously if well written etc. can help but the problem is email overload which happens even in your own mailbox--stuff can get overlooked. On email, it is often difficult to determine for staffers whether or not the person is a robot, someone from outside of the district who obviously is less important than someone in the district, a crank, etc. The weird email monickers that people use can also be a problem if
[email protected] is sending something, the staffer assigned to garbage duty might just not read it. The problem has only gotten worse as email is the new junk mail with spam, idiots, and people passing along chain emails, cluttering up inboxes.
Instead, I would propose using technology in a different way via epetitions or social media groups, or whatever, to provide concrete evidence that these are real people and real voters in a district tha have an opinion. Then print out the epetition/social media likes/posts/ etc. comments (there are multiple sites that do this) and results and present that to the local district office in printed form (with links so the staffer can verify the material). You have a petition with 1000 voters or more from the district, you will get some attention to your cause because the rule of thumb is that each might actually affect the vote of 4-5 people so the interest might be 5000-10000 votes affected by the issue.
Last, but not least, phone calls. In my opinion, at the federal level, these are less useful and often it is difficult to get through during peak votes. Some congress critters have apparently shut off calls even in high emotion events. If you must if time is of the essence such as unexpected amendment etc., you can try the national office but also try the local office as they will relay it. Better yet, most offices still have the fax machines to send a written message as an alternative because you might only get a moment to say "Yes I support HR 7328 or no, I do not support HR 7328--thank you", if the controversy on legislation provokes massive response. Remember you are also competing against interest groups that can afford phone banks and I suspect even paying shills to call in.
Depending on the size of the state or local government, some of these tips are more or less applicable but the officeholders can be more accessible than federal officials and fewer staffers to shield them from the citizens. Direct contact is almost always best unless you present a poor image of what you are trying to do. People are judged on appearances and behavior--deal with it accordingly. Written, as mentioned above, is also useful depending on the format. Avoid stale talking points derived from the media or mass mailers of your organization--chances are that the officeholder has heard it all before. Instead, speak from your heart and mind and be informed on the issue that you are trying to address. Remember you are trying to persuade someone so hectoring them, boring them, threatening them is pointless whether it is a staffer or the candidate themself. And last but not least, there is strength in numbers so the social media/petition approach where you are the bearer of actual voters and their opinions will strengthen your approach.
For better or worse, winning or losing, I have been involved in campaigns at the local, state, and federal levels and public policy issues at these levels on and off for nearly forty years. and these are my observances and one might say my professional opinion. As usual, YMMV and you might have a different take but remember, as much as it might seem that politicians are aliens from another planet at times, they are human too. They have feelings, likes and dislikes, and pride as well--so use the Dale Carnegie approach rather than treating them as an instant enemy, an excuse to vent over everything wrong with the country, or a dummy. The old saying if you want to friend a hog, then you gotta friend them back is true.