Discretion: This is between you and the other person, there is no need for everyone in the world to know your business. This person is not your girlfriend or boyfriend so talking about them incessantly will eventually get back to them and most likely result in a termination of your little agreement.
Transparency and Honesty: In most cases, the ongoing situation is quite clear and doesn't need to be discussed (who likes the awkward, "So I'm not really looking for anything serious..." conversation?) but sometimes it has to be had. The only way, seriously, that this situation works out is that both people are fully aware of what the deal is and are ok with it. The second something changes, you need to be honest about it.
Third Parties: Remember: this person is not your girlfriend or boyfriend. While they should respect you and make you feel comfortable they don't owe you anything more than what is established. They are allowed to see other people, it is none of your business if they do, nor should you ask them about it.
Don't be selfish: Just because you aren't dating the person, doesn't mean you should be an inconsiderate lover. This is, after all, an agreement between two people that they are both supposed to benefit from.
Be responsible: Hopefully I don't have to explain this.
So key take aways:
1. In regards to sleeping over, do what you feel comfortable with, though staying over is optimal. Never make someone leave.
2. Flirting/banter/casual conversations should happen. This isn't a stranger and you don't want someone to feel used.
3. Don't talk about it
4. Make sure you're on the same page
5. Be nice and respectful.
6. This isn't a booty call, therefore some hanging out is usually required. Drinks/dinner is not a big deal, and if it is, then rethink the situation and person.
7. Don't have unrealistic expectations, it is what it is.
8. Who else they're seeing is not your business. Just like yours isn't theirs.
9. Both parties should enjoy this, so bring your best to the bedroom.
10. Be responsible