Please don't:

• Plead for votes in the title of your submission. ("Vote This Up to Spread the Word!", "For every upvote, I'll give this starving child one grain of rice," etc.)
• Conduct polls using the title of your submission. Instead of "Vote up if you're male, down if you're female", say, "Are you male or female? (Vote in the comments)" and then post two comments, "Vote for this if you're (male/female)"
• Send out IMs, tweets, or any other sort of message asking people to vote for your submission -- or comply when other people ask you. A link should get points for being good, not because the submitter is part of a voting clique.
• Mass-downvote someone else's posts. If it really is the content you have a problem with (as opposed to the person), by all means vote it down when you come upon it. But don't go out of your way to seek out an enemy's posts.
• Downvote opinions just because you disagree with them. The down arrow is for comments that add nothing to the discussion.
• Announce your votes to the world. Comments like "dumb link" or "lol, upvoted!" are not terribly informative. Just click the arrows.
• Linkjack stories: linking to stories via blog posts that add nothing extra.
• Post hoaxes. If snopes.com has already declared something false, you probably shouldn't be submitting it to Reddit.
• Flood reddit with a lot of stories in a short span of time. By doing this you monopolize a shared resource - the new queue.
• Complain about too many stories on a particular topic.
• Complain about a story being old. Reddit is about interesting stuff, not new stuff only. Just hide the story.
• Complain when a duplicate story finds more success than the original. Posting a link to the original is okay, since earlier comments may be of interest.
• Complain about downvotes on your submission. Every story and comment gets at least a few downvotes.
• Moderate a story based on your opinion of its source. Quality of content, not location, is what matters.
• Write titles in ALL CAPS.
• Editorialize in the headlines or be overly-sensational.
• Link with tinyurl or similar service. There are few reasons to hide what you're linking to, and most of them are sneaky.
• Be rude when someone doesn't follow Reddiquette: Just point them here politely. And keep in mind that these are just guidelines.
• Create mass downvote or upvote campaigns. This includes attacking a user's profile history when they say something bad and participating in karma party threads.
• Reply to comments when you are really trying to address a sub-section of Reddit. Get a blog or start a new post, don't hijack other people's comment threads to make your point if it's unrelated to the comment.
• Take upon moderation positions in a subreddit where your profession or place of employment could pose a direct conflict of interest to the neutral and user-driven nature of Reddit.

Please do:

• Keep your submission titles factual and opinion-free. If it is an outrageous topic, share your outrage in the comment section.
• Moderate based on quality, not opinion. Well-written and interesting content can be worthwhile, even if you disagree with it.
• Look for the original source of content, and submit that. Often, a blog will reference another blog, which references another, and on down with everyone adding ads along the way. Dig through those references and submit a link to the creator, who actually deserves the traffic.
• Post to the most appropriate reddits possible. If you're submitting an article about U.S. politics, for example, it would be better suited for /r/politics than /r/reddit.com.
• Search for dupes before posting something. That said, sometimes bad timing, a bad title, or just plain bad luck can cause an interesting story to fail to get noticed. Feel free to post something again if you feel that the earlier posting didn't get the attention it deserved and you think you can do better.
• Put the file type at the end of the title if it is something other than html or text, like [pdf] or [video].
• Link to a direct version of a media file when the page it was found on doesn't add anything of value.
• Link to canonical and persistent URLs where possible, not ephemeral pages that might disappear. In particular, use the "permalink" for blog entries, not the blog's index page.
• Consider posting constructive criticism / an explanation when you downvote something, but only if you really think it might help the poster improve.
• Report any spam you find.
• Read the new page and vote on it. Regard it, perhaps, as a public service.
• Actually read an article before you vote on it (as opposed to just basing your vote on the title).
• Feel free to post links to your own content (within reason). If that's all you ever post, and it always seems to get voted down instantly, take a good hard look in the mirror -- you just might be a spammer.
• Submit links as links. If your post is in reference to a website, put that URL in the link field rather than linking to it from a text post. Using the link functionality of reddit makes it easier for people to see what you are trying to share.
• Link to original embeddable content where possible. Youtube videos (for example) can be viewed within reddit, youtube videos embedded on your blog cannot. See a list of embeddable sites here
• Use proper grammar, capitalization, and spelling.