Here's a few common topics:

"please use the original title, unless it is misleading or linkbait" (title rather than a summary or ad)

"If a story has had significant attention in the last year or so, we kill reposts as duplicates. If not, a small number of reposts is ok."

"Show HN is for something you've made that other people can play with. HN users can try it out, give you feedback, and ask questions in the thread."

(Also, it's not stated because it's self-evident, but: don't do disingenuous things to focus attention on a submission, such as submitting it from multiple accounts.)