Personally, I would recommend being straight with your wife sooner rather than later, because laying out hints will eventually, in retrospect, come across as manipulative dishonesty. Look inside yourself to assess your own motivations for delaying being straightforward, and it's possible you will discover that you may be doing this out of a desire to avoid conflict. Well, if conflict is going to occur with your wife around the topic of plural marriage, you may as well dive into that as soon as possible, in order to begin the time clock of whatever it is she's going to have to go through in order to process the new information along the way toward acceptance and ideally embracing the possibility within her own life. If you don't think you're ready for that, or even if you don't think your
wife is ready for that, it still comes back to the fact that, as the man in the matter, you are the one who is 100% responsible for everything that goes on in your family -- so if she's not ready, the place to look is where it is the case that
you're not ready to move forward, and that's likely to be related, as other men have mentioned here before, to the degree to which you have established yourself as the patriarch (not the lord and master, but the true leader) of your household and your marriage. I say all this with great humility, because my own search for a second wife is predominantly on hold and has been for over a year now, because I realized that I wasn't sufficiently the head of my household and had other, much more crucial, irons I needed to get into the fire before my household would be ready for expansion.
The joyful thing is that tackling becoming a true patriarch head-on has produced so many blessings in my marriage and family that, should I never have a plural family, just becoming the true leader of my household will have made getting involved in all this way beyond worth it.
At some point,
@Reuben, you're going to have to cross the bridge of addressing the fact that your marriage began with a promise to your wife that she would be the only one. If she's like almost all women, she's got something akin to a death grip on that promise. No amount of applying philosophical W-D40 is going to make a dent in the corrosion that has encrusted that death grip. In fact, I tend to disagree with almost every reason people give for why Christianity in general is so vehemently opposed to plural marriage. In my perception, it not only has nothing to do with Scripture or religious beliefs or fear of eternal damnation, nor is even the whole fairy-tale princess fantasy the driving force behind the resistance or even feminism. The fundamental momentum behind unwillingness to consider the possibility that polygyny should not be considered acceptable is plain and simply the nearly universal human desire to be popular and/or receive active approval from one's community. Our culture has not considered polygamy acceptable for so many generations that myths like the definition of marriage always having been a union between one man and one woman can be voiced without even a hint of irony. People want to be Liked, and people want to avoid being scorned, and being on the side of polygamy is just too much of a no-no. When you add that to your wife living in the land of assuming that she long-ago already won her personal husband sweepstakes, this subject is not going to be one you can just gradually soft-soap her out of like you might have been able to about getting that new grill or buying tickets to whatever concert you wanted to attend.
Please don't hear me in any way as saying that you should give up. Please don't give up. The process is doable. My wife nearly left me over all this, but now she's even kind of looking forward to the benefits of having another woman around the house and in her husband's bed. But what so many of us have learned is that this is a process that demands manhood. It simply will not work to take this the way Dr. Phil might advise one to approach it.