PART 1 REPLY...
it all depends a lot on what you want in this casual relationship. i'm going to assume that you are referring to a FWB (friends with benefits) relationship where you become friends and fuck. there certainly are relationships wherein you only meet up to fuck and then part thereafter. since the OP mentioned "ongoing" then i'm going to reply as such.
i had a few handfuls of FWBs growing up and my wife almost became another FWB but once we got to know each other more, we both realized that we had found each others' match and decided to go exclusive as boyfriend-girlfriend but would still fuck other people with consent from each other. i want to say that what happened with my wife is a rarity and don't expect any FWB to mature into any type of relationship other than a purely physical one. my wife simply fit my bill of a woman so perfectly that i couldn't pass up the chance to make her mine (and she couldn't pass me up either).
here's a couple of guidelines to follow:
BE HONEST AND VERY CLEAR ABOUT YOUR INTENTIONS: when it comes to the relationship, you have to be honest about what you want and what you expect, period. your partner should also do the same. if both of you cannot do this for each other then don't even bother having a fwb-relationship because sooner or later someone is going to get hurt, attached, and/or misunderstood. this will cause friction in the relationship and things will start going south really fast.
DEFINE THE BOUNDARIES OF THE "FRIEND" PART: an fwb-relationship is as much about being friends as it is about being fuck-buddies however set the boundaries of where your friendship ends. when you meet up with your fwb, you're gonna chat and catch up on life-events with each other before the clothes come off or even afterwards however you have to decide what you can and cannot talk about. for example, one of my rules with all of my fwb-ladies was that we didn't talk about things going on within our families or our careers. we both knew general information about family/career but we never shared anything detailed or personal. i knew she had 3 siblings and her mom was divorced but no clue why her mom was divorced or what her siblings did for a living. another rule was that we would not ask each other for help within our own lives. i was not there to save her and she was not here to save me. overall, this is a very grey area and only you can decide how far you want the friendship to go. the danger here is that the more you get to know a person, the more likely you may start to form an attachment; and that's a big no-no.
DON'T SHARE YOUR FWB RELATIONSHIP WITH ANYONE: probably the biggest mistake anyone can do is to let the cat out of the bag to a friend or whomever that you have a fwb-relationship. this applies to both partners. the issue here is that we live in a small world and if in the off-chance your FWB can be connected back to you in any way, it will ruin the relationship and make it awkward. i made this mistake with my very first fwb-lady. i boasted to a buddy of mine about who i was fucking with and about 3 weeks later, found out that she was a cousin of one of my buddy's sister's best friends. the word got out and she was slut-shamed by her friends and family. long story short, we quit fucking thereafter and i've never spoken to her since.
DON'T FALL IN LOVE OR GET ATTACHED: you would like to think this was a given, right? this happens more than not. we're humans and we're social creatures that crave that sense of belonging and acceptance; couple that with hot-good sex and you've got a formula for puppy-love at the minimum. this applies equally to both men and women. always remember that this relationship is built on physical sex and nothing more. the friendship is only there to keep it from being clinical; not to mention, making it a little bit more humane in that you have a person you can trust who is safe and clean. if you feel that you are developing feelings for your partner or you notice that your partner is starting to have feelings for you, then you need to either check those feelings away or end the relationship; just walk away... now some of you are thinking "i can turn this into a real relationship and make him/her mine"... WRONG! remember that the relationship was built on the need to find a partner to fuck. the relationship revolves around fucking each other. if you decide to go down this path, please know that you are on a very slippery slope heading towards thin ice. it can work but i don't recommend it and i don't have enough time to write how to make that work or how it can all fall apart.
DECIDE IF YOU WILL BE EXCLUSIVE FWBs or NOT: this is a touchy subject for both men and women however it needs to be clear from the get-go; no exceptions! this can cause serious damage both mentally and physically. this was my #1 rule when i was hunting for fwb-ladies; we only fuck each other. the notion is that you want to find a friend who wants the same thing so that you both can benefit from each other. if she is also fucking other guys as well as myself then i'm outta there. i have nothing against her doing this and she is more than free to do it but i don't want any part of it. it's not so much a man-thing in relation to being possessive; it's about getting STDs and whatnot from her other partners that i don't know about and/or trust. now there really is no way for you to know if your FWB is fucking someone else on the side so that's where the "friends" part comes into play. you should have a certain level of trust and know your FWB well enough to a point where when they say "you're the only one i'm fucking" that you actually can believe them. i've never been as ass with this rule however i've always made it clear that if they wanted to fuck other people then go ahead, just let me know so i can go find another exclusive FWB.
DON'T GET HUNG UP ON LOOKS OR FINDING THAT PERFECT PERSON: remember that an fwb-relationship is about fucking and nothing more so there's no need to set your standards on high. if you're looking for that 5'8", blonde, blue eyes, with 36C boobs weighing in at 120 pounds with clothes on then you're gonna be waiting a very long time... lols. the fact is that if she exists, she's getting hit on every day by men across all the spectrum; even good men who are well-intentioned and ready to love/marry her. women like this are far and between in the fwb-world. if you do find one, she is either very career-driven and have no time for a real love-relationship or she's had a bad run with men and has given up. either way, there's dynamics there that needs to be accounted for (and i don't have time to explain it here). what i recommend is to find a partner who is height-weight-proportionate, who has very good hygiene, and a strong sexual prowess. someone that knows how to take care of themselves physically. usually people who fall into this category are, for the most part, good-looking people if not average-to-above-average individuals. this is the perfect range to be in. these people will not be model-ready to be on the face of magazines or commercial ads but pleasing enough on the eyes. for me, it's always been about cleanliness and hygiene. if a woman knew how to take care of herself overall and had good skin all over her body then she was a potential FWB to me. of course, i would reciprocate all of my expectations and be very clean overall myself too. don't get hung up on the minor flaws either... if i found a woman with nice skin overall, decent body shape, but she had crooked teeth, it was not a deal breaker as long as she kept her teeth clean. i hope you're getting the point here.