Assuming that you're talking about opening up a monogamous relationship into a one in which both parties can just date around. This decision has to be absolutely to the nth percent mutual.

If one side wants this and the other isn't on board 100%, and that includes "faking enthusiasm," then it's not going to work.

Beyond that, establish very specific rules and "veto powers," and you'll have to learn to trust each other near unconditionally. Keep that jealousy in check at all times. And if it gets to be too much, call it quits and re-evaluate the relationship.

I need a better definition of "opened up". If you meet someone and it is going well, what more does one need? Why open up the relationship to failure?