Top 10 Tricks to Make your Villain Stand Out

This is from a couple of years back, but it is still good advice. Via Logan Bonner the excellent blog/website Dungeon Mastering:

10. Has Boundaries. Your villain will be more well-rounded if there are some things he or she won’t do.

9. Keeps Poking Old Wounds. Take note every time the adventurers fail to stop the villain, or let a part of its plan succeed. A good villain exploits those embarrassing moments

8. Signature Symbols. One of the best ways to make the villain’s presence known is by reusing symbols and calling cards.

See the full list in the article at the Dungeon Mastering website.