As a standard thing to be said to new posters, remember to search for similar topics in the search bar in the top right of the screen. You can punch the magnifying glass as an advanced search, and pay attention to whether you are searching a thread, a forum, or the whole forums. Also, searching is Boolean AND (as opposed to Google's OR) - every word you punch in has to be in a post in the exact way you spell it for it to return a hit, so the less words you search for at a time, the more likely you are to get a hit.
If you find threads with very similar themes to the ones you are going to post, we ask that you either read and incorporate those other topics into your own if your own diverges significantly enough, or that you contribute to already-existing threads if they are similar enough. There is no penalty for "thread necroing" so long as you are doing more than just saying "bump", and are adding something. This is because, over time, some topics are brought up again and again, and people grow tired of arguing over the same topics that get argued into a stalemate time and again. By contributing to an already-existing thread of some substance where those arguments have already taken place, things can move beyond those arguments.
In this vein, it is a good idea to avoid "My List Of Suggestions" types of threads. Threads should have topic lines that let you tell what's in them, or else people will assume it is nothing but every idea that came off the top of an excited poster's head, and those are typically going to be many of the same ideas that came off the last person's head. Posting long, complex topics with multiple sub-topics under a single thread is fine, so long as it is guided by a reasonable topic. If you have plenty of thoughtful things to say about multiple different suggestions, then odds are, you'd do better with multiple threads.
Likewise, it's worth looking at the
devpages, and
Toady's long-term goals, to make sure that you aren't suggesting something Toady was already planning on doing, anyway. Starting a thread to go into detail about something Toady is planning is fine, but just "we sould do X" when X is already on the devpages becomes redundant.
There are a great many suggestions on the suggestion forum, and so there is more value in arguing through and hashing out the details and implications of a suggestion, and what this will change about the game, than there is for simply throwing out a concept that the game might incorporate. Most suggestions will not be acted upon for years, if ever, and as such, there is a far greater value in taking the time to fully explore every avenue of discussion with a given topic than there is in simply brainstorming as many individual ideas as one can.
The Suggestions Forum is based upon a give-and-take of ideas, so if you want to be listened to, you have to be willing to listen to others in reading what has been discussed before you, and responding with constructive comments.