Be a really good listener: guys don't just want a shoulder to lean on, they love good ears. This way, he'll tell you things he wouldn't tell his main chick and you can slowly ease your way into his life.
Don't be too clingy, show him your worth: please for the love of Christ, don't be blowing up his phone eery minute or showing up everywhere he is! Give him space to pursue you and show him that even tho you're the sidechick, you still know your worth.
Be a fun loving person: chances are his relationship is already boring, that's why you're his sidechick in the first place. Now, the trick here is to be an adventurous person ALL DAY, EVERYDAY. No dull moment when you're around,not even for a second. Doing this, he'll start wondering why his mainchick isn't like you and begin to question his relationship.
Your sense of humour has to be LIT: see ehn, I don't care if you have to start reading books and articles or watching documentaries in order to become funny. But girl, if you have a good sense of humor that can make any guy laugh or engage with you on any level, trust me, your boyfriend's girlfriend won't stand a chance against you. You'll instantly become his happy place and he'll keep coming back for more.
Be friends with the mainchick: this one is a bit tricky but it works the magic. You have to be smart about it tho. Make acquaintances with the mainchick come of as just another girl that's friends with her boyfriend. This way, the aunty won't suspect anything or perceive you as a threat. But you need to be subtle with it and not get close to her. Just hello/hi is fine. This way she may trust you with her man.
Text/call him whenever he's with his mainchick: this works magic because chances are your boyfriend wont take your calls or reply your texts whenever he's with her. But the more you keep texting, whenever he's with her, the more the mainchick will start suspecting him. If he denies anything going on Bettye two of you, she will suspect him more, become paranoid and starts nagging. And you know guys hate nags. Lol.
Start doing all the things the mainchick is not doing multiplied by 2: whatever it is he complains his mainchick isn't doing enough or doing at all, please feel free to do them and add extra. Bring out his big self and manage his ego so that he begins to think you're his God sent. After all, you're only being a good friend,right?
Let your sex game be lit AF!! This is one major key! Don't let anyone deceive you by saying sex will not keep a man. Trust me, if your sex is fire, he'll forget you're his sidechick and moan your name during sex with the mainchick...and you know what that means, right?
Understand you're not the only girl in his life, but feel free to act like you are: don't give him room to mention his mainchick around you. Give him an ultimatum. Make him choose between yourself and his mainchick and if he's being indecisive about it, threaten to break up with him for good. You do not need such negativity in your life, sis. Once he sees how serious you are, he'll b forced to make you official.
LEAVE HIM ALONE!! GO FIND YOUR OWN BOYFRIEND!!! look at you. So you 2018 goal is to steal someone else's boyfriend,huh? May God forgive you. Lol
Stay woke, y'all.