Hints can be a valuable part of a good puzzle, but there are many examples on site where improper use of so-called 'hints' really degrades quality of the individual puzzle or the site as a whole.
The purpose of this meta-site post is to collect (and vote upon) general advise of how to use - and how to not use - hints in a puzzle.
Hints should be supplementary
A person should be able to solve a puzzle completely without ever referring to them.
If your puzzle needs a hint to be solvable, it is not a hint but an integral part of the puzzle! Don't call it a hint. Add the information to your main puzzle.
Hints are not for making a puzzle interactive
Never-ever use hints to create an 'interactive game'!
Do not post a puzzle with the intention of adding hints as parts of the puzzle are solved. Instead, structure your puzzle in a way that all the information can be given up front. Although playing 'games' with other users can be fun, puzzling.SE is not the place (there are chats and forums for this). Don't be surprised if an interactive puzzle get downvoted or closed, regardless of how "good" the puzzle is.
Content on PuzzlingSE (and SE in general) is meant to last.
Imagine yourself coming across your post in two years time. Just the question. What would you think about it? Would you like how it is presented and what it contains? Only if both answers are YES have you managed to produce on-topic-content for the site.
Hints are not 'patches'
If your puzzle is 'broken': Edit your post and fix the problem, but don't add hints!
When creating a puzzle, one can easily make a mistake or realize at a later point that not enough information was given. This is okay. We are all learning here.
When you want to improve your puzzle, do so in a way which makes the complete posting look good and complete to a new user. Edits don't have to be cumulative and you don't need to keep obsolete text in the post - that's what accessing the "edit" page is for.
If some people have already given answers which become invalidated by your edit, you may want to add some comments to their answers indicating that you have changed things.
Additionally, you can also keep a log of changes you've added after initial posting (example).
Hints should be spoiler-tagged
Prevent 'accidental' reading of hints by putting them in spoiler tags.
(Some help with formatting spoiler tags can be found here)
Hints are (often) not needed
This may sound surprising to you, but puzzles are generally better without hints.
Many hints are added because the OP is impatiently waiting for some response and deems the lack of immediate responses be caused by 'difficulty' of the puzzle. That's wrong. There are many reasons why a puzzle may not receive immediate feedback, and one of them can simply be that it is indeed a very good puzzle! People want to work on it and then present the complete solution.
Few things are as annoying as puzzles getting hint after hint within hours (if not minutes!) from their posting. Don't add a hint before you get some request for one by multiple users. If your puzzle is good then it will need time to be solved, and people not always post their work until it is finished. Don't ruin their fun and your puzzle by "over-hinting" it until a crash-test-dummy could solve it!
Just imagine you're solving a puzzle, have spent a couple of your hours on it and feel really proud to have cracked it. Now you want to nicely summarize it as an answer post just to realize that dozens of answers are already there, because the crucial thing of the puzzle has been spelled out in red letters in a "hint". How frustrating would this be?
Also think worldwide: It takes ~24hrs to have 'all regular users' spot your puzzle, simply because they are distributed over all time zones. Also not all weekdays show the same amount of activity, so maybe it takes a couple of days before all potential puzzlers have even seen you puzzle.
Rule of thumb
It is possibly a good idea to wait 'at least 1 day' before adding any hint. Better still: Don't add anything until you've received comments that the puzzle is 'too hard' and ideas would be welcome. Some very good puzzles remained "silent" for months, but people were working on it! If you're not sure, rather post a comment asking "Should I add hints?" before adding them.
Hints are no means of pushing a puzzle
Don't edit in hints just to push a puzzle up the activity list.
We all know that your puzzle is great and requires attention, but pushing it in our face by 'activating' a puzzle again and again with little edits in form of hints is impolite, annoying and not helpful.
