Firstly I am very excited that this in my first post as a result of a request, which you can e-mail me for. Some subjects come very naturally to me, but this one is trial and error, (not just my own trials either, I am a great listener and observer). Firstly, you must not develop feelings other than friendship for the partner. Secondly, be as open about the situation as possible, lay ground rules for the arrangement and be clear about the intentions. Thirdly, long term doesn't work! Keep it short, with a time line for ending things in mind, whether this is accomplished by having a few partners on rotation or making it just a 3 week fling.
If you break rule number one, make a clean break as quickly as possible as there is no going back. One man fell hard for me after only a few times together, and proposed that we start a family based on feelings of lust. I severed things immediately afterwards but it took a very long time to even get a friendship back. On the other hand, I have done the falling and first hand I can tell you that it is necessary to back right off and ensure those feelings do not ever come through. In both my experiences the communication was not clear, and there was no end date, so there was fancy footwork and fast talking needed to ensure that the friendships remained in tact.
We are all adults and there is no reason I see that sex cannot be a natural expression of being human. But please show sympathy if the partner shows any feelings outside of your shared moments of lust. Either try and communicate clearly point number two about the ground rules and your current needs, or you will have to end things very quickly and swiftly. Dragging out anything is cruel and there will be no way to ever run into the person again, or worse, a nice little stalker might be the result.
The only exception to following the 3 rules above is if with open communication and both parties on board, you try and make an actual go at a relationship. If not keep it simple, and know that however hard, it will end. How it ends, is all up to both partners and keeping open about the situation only. Please never lead a person on to hope that this can continue long term when it can't.