I can only speak from my experience as FW and dating... That said, here are things I learned.
Don't try to displace the first wife... It seems obvious but it might as well be said. The circle is expanding, not shifting. If in the beginning the husband turns to his FW with problems, don't take it personally, it is just born out of years of him leaning on her and is not a reflection of you or your relationship with him. It takes time to build relationships, it doesn't happen overnight. This was a huge issue with a potential we were seeing as she kept whining that "he preferred me". Well, yes, I suppose he did but at that point we had been together 15 years and it was a comfortable thing to turn to each other. She took it personally that she did not instantly have that but never once did she try to actually build a relationship with hubby. Ultimately, she wanted her half and most of mine too and I wasn't giving it, which frustrated her since she very much had an attitude that I had my time so I should just disappear in the shadows and hand her my family and let her catch up. Yeah, doesn't work that way.
Communication... Instead of going off in a huff, try communicating with future hubby and FW about your feelings, needs, dreams, etc. Of course, be reasonable. Example: Don't be mad that hubby took FW out for her birthday to dinner and start railing at how unfair it is she just got a date night. (Yes, this really happened) Men are notoriously NOT mind readers and you have to spell it out. Instead of fighting the FW and getting huffy, try instead talking to her. Chances are we can empathize and understand what it is you are needing and relay that appropriately to the husband. Also, here it should be noted that during the courting phase, you need to build a relationship with the FW and not just the hubby. I will not lie, if I feel slighted and "dumped", I will make life difficult for both hubby and potential and ultimately, that relationship will not last. You want FW as an ally, not an enemy. Trust me on this one. Men who have been in a long-term marriage are not going to dump their first wife and run off with the new wife. Yet, some of the girls do seem to fantasize about this happening. (Note: This is not personal or directed at you, just a general observation)
Even if the hubby marries the woman without the approval and blessing of his FW, I guarantee you not having a good relationship with her will make your life miserable. Even if FW decides to leave, especially where there are children involved, she will "own" him and take more of his attention at that point. Simply put, his guilt and legal obligations will keep him tied to her for a long time and she is not gonna go away quietly leaving the couple to wedded bliss. Thus, the absolute goal of an incoming SW is to support the marriage of the FW and hubby and work towards a goal of living harmoniously. Just as you want the FW to accept you and support the relationship with hubby, so should you do for the FW. When things get rough, she will defend you, stick by you, and advocate for you. Do the same for her.
Okay, this sounds kind of gloom and doom and I don't mean it to. I guess my biggest point is just always remember, there are actual people with actual feelings involved. Put yourself in our shoes as much as you can and use empathy. I know there are some on this forum who believe the FW doesn't get a say, but that is just foolishness on their parts because in the end, FW does have a choice to leave or go and the courts will support her in that. You do *not* want to be involved in a nasty divorce battle and watch a family break apart and a man destroyed because he was thinking with something other than his brain. Ultimately, you would be the biggest loser in that contest sadly. Single girls need to protect themselves in a lot of different ways, and this is one of them. Make sure the marriage is stable and the FW is in agreement. God will not (NOT, no matter what anyone trys to tell me) bring a husband to the conviction without also bringing the wife. If the wife is dead set against Plural Marriage, expect nothing but troubles and heartaches. Further, if a man waits too long to introduce you to the wife, RUN... he is about to dump something on his FW and it will be UGLY. Again, RUN FAST.
It is commonly known that women have the power to hurt each other far more than a man does. Every emotion you have and evil little thought (we all have them, LOL), be aware that so does FW.
Oh, one other thing... Respect her experience. If she tells you hubby hates broccoli, trust her and don't test the theory thinking your broccoli will be better. She learned from experience and she knows him well... let her teach you and learn from her... it benefits YOU most of all as it helps you develop the relationship with hubby and gives you brownie points with FW for listening to her and making her feel like she matters, even if it is just how he wants his socks folded.
~Becca