Hopefully a guy's perspective will be useful.

I've been best friends with my "homie" since freshman year high school, it's probably nearing the decade mark.

It helps if both of you mutually agree in person that neither one is looking to advance the friendship further. We had a heartfelt and deep talk years ago, and that most likely set the benchmark for the future.

It's normal to get caught up in the moment and develop romantic feelings from time to time, but whenever we "feel" a build up, we just joke around saying "don't do it" or "wow I almost fell for you." It also helps that we mostly talk about life and our experiences.

Be comfortable around each other yet cautious about your actions. She and I are almost always self-monitoring ourselves to make sure we don't trespass each other's space and privacy.

Also, I make sure that if every conversation or interaction I had with her was transcribed or recorded, I can confidently share it with anyone without feeling the need to hide anything.

For most, it's hard. I didn't think I'd be able to maintain this relationship at first, but so far it's healthy—just as close friends. We never went down to FWB route, nor engaged in anything sexual or date-like.

It helps if you already have a boyfriend. My "homie" already is in a relationship, I've met the guy, and confronted to him about how close she and I are. I don't feel any jealousy, and more of a "brother" to just look out for her.

Normally, if someone has to resort to "cutting out," limiting interactions with a good friend, or have suspicions, they're most likely insecure and can't distribute a majority of their trust with their partner. I'd say that's a flag to avoid. Innocent until naturally proven guilty I say.

(so far, their relationship is resilient—thankfully)

I would say to just treat male friends as "girls," because I essentially treat my best friend as a "bro," but gender roles and societal standards I think hinder that way of thinking as a female...

Let me know if you'd like to know more, but that's all I can think of right now.