I can't help but think Raga's whole post was addressed to me, considering how specific it seems, but I do also think it offers everyone opportunities to learn from mistakes Raga and I have made, and it provides basis for an interesting discussion.
So I am presenting advice from the other point of view, the point of view of the person who opened the relationship, giving advice to the mono partner.
Don't lie to your partner
This one remains the same for the mono partner. Honesty and communication are fundamental.
Don't use emotional blackmail
Do not go along with something for fear or losing your partner, and don't lie about being okay with a relationship when you are not. If you are honest with your partner, they might indeed leave, but that will save you both some trouble, as if you're not comfortable with the lifestyle to begin with, it will end badly.
Don't assume your partner is trying to manipulate you, either. If they tell you they don't trust themselves not to cheat, they might simply be following the previous rule. They might say it from experience, too, having tried hard to be mono for years and being constantly tempted to cheat, resisting because they don't want to betray their partner, but feeling like they won't be able to resist much longer and trying to find an ethical way to go about it.
In my case, I felt it was more fair to give Raga all the data, so that he knew what to expect. I realised it meant risking to be dumped over it, but I was willing to take that risk for the sake of honesty. I felt it would have been despicable of me to lie ("oh, if you don't want to try, it's fine, we'll go back to being mono") and then cheat. I felt he deserved better, and I wanted him to make a choice with all the information in mind, so I didn't hide any of it.
At no time did I feel I was manipulating him to force him to accept poly. Quite the opposite, I felt I was giving him a clearer choice, that he knew it was A or B, there was nothing grey about it, and I was willing to get dumped over it because I wanted to be true to myself and honest with him. We all have deal breakers, you need to be honest about them, both to yourself and to your partners.
Your partner's fears are legitimate,
Always remember that this is something your partner didn't want
Remember that while it is scary, new and weird for you, it is also new, weird and scary for your partner. They took a chance when coming out to you, they risked losing you and alienating you. They also risk alienating their friends and family if you refuse, leave them and give it as a reason, and being labeled a cheater even though they tried to do the right thing. Even if you stay, as the poly partner they will be seen as the bad guy to everyone you come out to. They took a big leap of faith and that means a lot about how much they trust you and how much you matter to them. You didn't choose for them to be poly, but they didn't choose to be poly, either.
Also remember to state your mind. Don't fall into the trap of "if they really loved me they'd realise I'm having a hard time". People aren't mind readers. Tell them how you feel and what you think would help. Be constructive about it. Saying "I feel like crap" is one thing (and it's better than bottling it up) but it accomplishes little more than making your partner feel terrible. Try using sentences such as "I would feel better about X if..."[/QUOTE]
After you find your second relationship
Remember that NRE is like a drug, so treat it like one
Same thing. Your partner, not you, will be on that drug. Remember it's NRE and doesn't mean anything about you. Let them enjoy it without feeling guilty (it's a necessary part of any relationship), but tell them if they're going too far and neglecting you.
Don't feel that the other person is "better" or makes your partner happier than you do, remember it's NRE, and you most likely had it too.
Think back to your own NRE with your partner, and realise that it isn't fair to expect restrain if you didn't show any. For instance, if you wanted to shout your relationship from the roofs, it isn't very fair to ask for your partner's second relationship to remain a secret from anyone who might know you. If your partner and metamour are willing to go trough secrecy for your sake, remember it was a lot to ask of them.
Give both relationships 110%
The flipside for this one is the same as previously: communicate. Your partner is under NRE, which is a drug. There is no way they will notice that you're feeling neglected if you say nothing about it. They will be in their own world. Make sure to tell them how you feel and be assertive. Communicate it talking about your own need instead of sounding accusing, that is, say "I would like to spend more time with you" rather than "you're spending more time with X than with me!".
Also remember that your vision might be skewed. You used to have your partner's full time and attention. You might get double the time and attention the second partner is getting, and yet believe you are getting much less, because you assume your partner is with your metamour every time they are not with you. They might spend most of their time alone.
They might also consider time spend with both of you to count for both relationships while you might feel it doesn't count because you weren't alone.
If you feel neglected and resentful when you're getting more time and attention than your metamour, you can't expect your partner to figure it on their own. Be open and you might realise from what they tell you that they're only spending together a fraction of the time you thought they were.
The bottom line remains, communicate! Whether you are getting the short end of the stick or not, your partner won't know you feel that way if you don't say so.
Your partner bending over backwards to accommodate is a very bad sign
I'm sorry, but I simply cannot agree with the advice above. "If your partner looks happy and looks like they're embracing the poly lifestyle, assume they're actually feeling neglected and don't know it, so you have to guess for them! Also, they're still mono but they don't know it either!"
I'm sorry, but if the mono partner doesn't realise it themselves, it is completely unrealistic to expect the poly partner to be the one realising it. So, be honest with yourself and your own feelings. If you aren't, you will suffer from people treating you like you're showing yourself instead of treating you like the part of you you are hiding.
Never give you partner cause to compare your relationships
Try hard not to compare relationships. If you feel your partner is doing something with your metamour they're not doing with you, tell them you'd like to do it with them, too. If in the end they do not do it with you, talk to them about it. Is it a deal breaker? Why is your partner treating the relationships differently?
Ultimately remember, though, that different people call for different relationships. Maybe you're a lot of fun to play tennis with but you're not so good with card games and your partner is the opposite. Or maybe the nature of the relationship makes things different.
For instance, to use a personal example, when I had a live-in relationship and a long-distance relationship, it became crucial to do things in the long-distance relationship that could help us feel more "together". Things I wouldn't necessarily enjoy doing with my LDR partner in person but that I did online because it gave a sense of stability, regularity and closeness the relationship otherwise lacked.
In this case, it wasn't the partners who were different, but the relationships. If they had both been live-in relationships, it would have been different. Sure enough, now that I live with my former LDR, I don't do these things anymore. They're not needed anymore.
Talk to your partner to figure out if things of that nature exist, as otherwise you might feel you're being treated as inferior when it might means your relationship already has the closeness and stability the other relationship lacks.
(To be continued)