I've never dabbled in poly myself (just open relationships, which have some similar mechanics but a fundamentally different mindset), so the following is limited second-hand knowledge/opinion, but seeing as nobody else has replied...

First of all I don't think there's a definite answer to your question, as 'poly' is an umbrella term for a lot of different constellations: does everybody date everybody or are people just allowed to have several unrelated partners, is everything fair game or are there things you're only supposed to do with your main partner (if there is one), how are new partners introduced, who can have sex with whom, do the same rules even apply to everybody involved, etc. There's no right or wrong, only "everybody involved is comfortable with this" or not.

I myself haven't seen many poly constructs work out mid- to long-term, mainly for the same reason that many open relationships eventually fail: not everybody involved was equally enthusiastic about the open/poly part, and/or as mindful of their partner(s) ad the situation demands.

Those I've seen succeed all have two things in common: very, very good communication and unconditional trust.

One part of this is what some call "brutal honesty" - you talk about everything that might affect your feelings for each other - even, and especially, the things that may hurt your partner(s)'s feelings. This obviously demands a lot from everybody involved. You also need to accept the fact that your partner(s) will have feelings that may be hurtful to you but are still valid and good for them. For example you need to be genuinely happy for the person you love the most in the world when they fall in love with somebody new and/or have had better sex with them than with you. That's tough on many levels. It also means that there needs to be enough trust to accept and overcome jealousy and fear of loss.

You also see how easily such a degree of trust can be abused. That abuse accounts for, hmm, probably 90% of all the failed poly and open situations I personally know of. It never works out when one partner just goes along with it in order not to (entirely) lose the other partner, which sadly happens alot and is not always obvious from the start.

Funnily enough, a monogamous relationship would profit just as much from that kind of communication, only a monogamous break-up is generally more of an obstacle than a shift in a poly constellation (both for personal reasons and because of what society expects), and so monogamous constructs can be of a much lower quality before they're deemed unsustainable.

I've also asked a close friend with poly experience what they think is important and will add their response here once they've replied.