Hey, glad you asked :D
Here are the current rules:
Competitive movesets only, not in-game (although by extension, a good competitive moveset will likely be good in game).
Specify the format/metagame that the set is for. Say whether it's for OU, UU, VGC, Monotype, etc.
Remember to include ability and EVs. Natures and held items are also good to have.
A bit of explanation is useful (not required, but a sentence or two is good).
If a Pokemon has multiple forms (e.g. Meowstic, Mega Evos) remember to specify which one the set is for.
One moveset per answer only, so different sets can be upvoted separately. You can answer multiple times on one question.
Make sure to check the current answers in case the same moveset has already been posted.
No big pictures in answers, please stick to one sprite max. We have sprites for Sun/Moon up now, they can be found here.
If your moveset is not for the current generation, it is helpful to mention which generation it belongs to.
Movesets for Pokemon that are not yet fully evolved belong on their final form/s' page. So, movesets for Scyther should be on Scizor's moveset pages. This is to prevent excessive clutter.
Make sure nobody else already posted the exact same set. Because this happens sometimes.
Your posts might be modified or removed if they do not follow these rules -- make sure you follow them!
How to change your username
How to add a user avatar
Why was my post hidden?
The points system
Flags vs Downvotes
PokemonDb Showdown server
Report minor site errors
Name Origins corrections
Location guide corrections