There's only one real rule, simply because there's multiple types that present in different ways and many of the things that are mentioned can show up in other conditions or none at all, but the main thing will still be there:
They don't give to you in the same way you'll give to them. Once you stop giving, they'll discard you or punish you in a way so you can return to what you were doing. It's all about them, as they need their self-esteem boosted because inside they feel like there's nothing there.
It's actually sad for both parties, and I have empathy for narcissists (I don't for psychopaths though).