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Read https://news.ycombinator.com/showhn.html. Point users to something they can try out for themselves. Don't require signups or permissions that aren't obviously needed. Avoid popups. Avoid overly slick website design and (especially) marketing language: HN users tune all that out and in fact it hurts you. Text and text-based layouts are good. Information density is good. Avoid super-large fonts and excessive pictures, they make you look lightweight. Put intellectually interesting details up front. If you're launching a company, corporate branding is fine; otherwise it's a negative, so tune it down.
Add a first comment to the thread with the backstory of how you came to work on this and explaining what's different about it. This tends to seed better discussion.
Make it easy to tell what the product/project is; otherwise the discussion will consist of "I can't tell what this is". Link to any relevant past threads.
Your primary mission is to engage intellectual curiosity. If you try to sell HN readers on your stuff, you'll evoke objections. Engage their curiosity and they will sell themselves.
Mention areas you'd like feedback about or open questions. Surprising or whimsical things that came up during the work are also good--they are unpredictable and that makes them interesting.
A little humor is ok; more than a little feels presumptuous. Don't be chummy, just answer straightforwardly. Don't address other users by their usernames (it's not the convention on HN and feels out of place). Don't introduce yourself more than once.
Don't say nice things about yourself or your work. It invites comeuppance. Instead, be humble or even mildly self-critical; then readers will look for nice things to say, and even when finding fault, won't make as big a deal about it.
Don't ask for upvotes. Our software ignores most promo-votes, plus HN users notice them and get mad. Especially make sure that your friends don't post booster comments or softball questions. HN users sniff that out a mile away and then we have to kill the thread.
Email us a link to your submission when it's up and we might be able to give you some help or make sure it doesn't get flagged.
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This originated as advice for YC startups but I always liked the pg/yc tradition of giving the same advice to everybody.