Because the man I adore and I don't actually have sex, I have a semi-regular thing with a friend, "TL."
He's someone I've respected for a long time, for his music and general social charms.
So *1. Respect, in as many forms as it can be felt.
However, we have absolutely different tastes in everything that actually matters, particularly humor, and not in a sweet "opposites attract" way.
*2. Open acknowledgement of the fact that y'all shouldn't be in a "real" relationship. This one's tricky, and it's where I think friends of mine have gotten heartbroken; has to be an idea that occurred to both parties independently. I know we'd just hurt each other if we tried to mesh. This might be tougher if this person is a close friend of yours. I have to have more distance than that for it to work as an ongoing delight.
*3. Admitting you care about their wellbeing is always good; no body wants to be thought of as a dispose-after-sexing tissue. But as far as feelings of attachment are concerned, I've always found it's best to keep those close to the vest until you're absolutely sure what you feel. I think bodies often want to fool us into thinking someone we're frequently co-bed-itating with are/should be an integral part of the den. That's sometimes true, but not always.
All my relationships, aside from my oh-gosh-current, have been defined as time limited, at least on my part. Somehow, never found talking about that point to be conducive to the relationship's continuation. I think it's about keeping careful tabs on how you're feeling about each other, and the confidence that understanding gives.