In order to increase the probability of your submission staying up, you just need to research for a few minutes before posting or before ordering a post from us, The tips from this article will greatly increase your chances, but do realize that subreddits aren't fair or protected by the free speech amendment. Each subreddit is owned by its creator, and optionally he might have assigned moderator status to one or more persons, offering various rights. Do start from a psychological state where you won’t get upset if you get censored or if a moderator removes your post because it somehow hurts his bottom line; maybe he is the founder behind a company that is your competitor. Where we can, we will help; we have moderator status in lots of subreddits (especially finance-, business- and cryptocurrency-related).
The number #1 tip is to actually read the sidebar rules first! You'll find them in the right part of the screen:
After this, you will usually know if you've selected the right subreddit for your post. If you need another one, use these tools to find it:
Before writing the actual post, especially the title, it might be a good idea to read a few posts from the chosen subreddit. See what people are writing about, and try to get a feel of the community from the comment sections. It’s useful to be able to relate to them by using the same communication style. Additionally, go to the "Top" sorting -> "All Time", and see which posts did best. How were they formulated? What specific subjects were they touching?
Don't try to post obvious spam, and if you did it in the past excessively check that the domain is not banned. An easy way to do this: first do your post in /r/test, check with another browser if you can see it, and then remove it.
If your post was fine but it still doesn't show up, do know that many subreddits run a bot named "automoderator" (or "automod") that's often badly configured. Reach out to the moderators through a message, and ask them to approve your post. We always do this ourselves when needed.
Good luck! Add a comment to this article if you have more tips yourself.