Follow the submissions guidelines. This is always good advice. Research the people to whom you intend to submit your manuscript. Also good advice, and anyone who can’t be bothered to take the effort to follow basic instruction and take information readily available off a website so they can address the right person BY NAME probably deserves everything they get. Or don’t. There’s plenty of advice on what to do and what not to do, for example here and here and another example of why you really should pay attention here. I bring these agencies to your attention as they’re the only genre agencies I know of. (Edit: And ONLY for that reason – I have no beef with either agency and only reason I’m linking to them is that if you, dear reader, are an aspiring SF/F writer then you ought to know they they exist and read what they say about submissions.)
However, dear agents and editors and people who write submissions guidance and then point fingers and laugh at those unable to follow it (Twitter, I’m looking at you), please have a little consideration for your poor aspiring writers. Let us suppose I am that person. There are a lot of publishers and agents out there to whom one might send a query letter. About forty to fifty the last time I paid attention. Many of them aren’t in the least bit interested in my latest manuscript, but I don’t know that because I already ruled out the ones whose interest obviously lies elsewhere. I know that almost none of you will be interested and I’m damned if I’m going to write to you one by one and wait, individually, for a reply (the last time I was doing this seriously, the average response time to a query letter was about two and a half months. There was a lot of variation in this and maybe it’s changed but I doubt it. And I’m quite convinced that someone out there still has The Thief-Taker’s Apprentice sitting in their slush pile, gently gathering dust) because then I’ll be waiting for about TEN YEARS before I’ve collected my full set of rejection letters. So no, I’m not going to single-submit as a general rule, although I’m not going to tell you that. And if I’m researching 40-50 agents and publishers while at the same time as indulging my remorseless muse at the same time as holding down a day-job that pays the bills at the same time as having any kind of life whatsoever, you’ll understand that a lot of my research is going to be carried out over the odd hastily-snatched twenty minute slot over lunchtime. And even then I’ll probably be trying to fit in two of you in each slot because otherwise it’s going to to take about three months just to get your names and addresses (EDIT: Neither the JJLA or Zeno demand this – the internet and those books on ‘how to get published’ that I’ve read often say that you should, though).
So, agents and publishers, let me offer YOU some guidance. You should aim to have all your information available to me in that one ten-minute slot. Otherwise I’m going to default to standard ‘best practice’ that I’ve picked up from sites better organised than yours. And that probably means I’ll do something wrong. And then you’ll reject me, and we’ll neither of us know what we’ve missed out on. Ask yourself, if you don’t like it, how many authors you know that are good at writing books. About all of them, right? And how many of those are good at following basic instructions? What about meeting deadlines? Being organised? It’s not that we’re different to the rest of the world, it’s just that we’re, well, we’re not project managers, we’re writers.
Ten minutes. Let that be the test of how clear your submissions guidance is. Then you can criticize us for not doing it exactly the way you want it.
While we’re at it, aspiring writers, this is probably worth a couple of your ten-minute lunchtime slots.
 Except for some of us. But we all have our personality disorders, right?