I would like to describe a proposal used in Green Mars, which is the book I'm currently reading. Even in the book, this proposal is only theoretical.
During a research seminar on a tropical island, the old mentor Fort poses the conundrum of how to deal with the world population once the world cannot sustain the population. As a Christian, I am against the use of abortion and most methods of population control, but there was an interesting population control mechanism that a main character, Arthur Randolph, of the book put forth.
"I would give everyone alive a birthright which entitled them to parent three-quarters of a child."(p. 82)
Now this is interesting. Suppose a man and a woman marry. Then between them they can have one-and-a-half children. Consider that they then have one child. From here the couple has a choice. They can either be content with one child or they may want another or maybe even more. If they are happy with one child, they can opt to sell their combined half of a child-right. Of course the next realization is that if you choose to lead a single life, or have no children, you can sell your full three-quarters of a child-right.
On the other hand, if the couple wanted to make more children(i.e. they could afford it, and had the desire), they chould buy another half of a child-right from another couple, and thus have a full child-right to themselves. Thus child-rights would be a commodity, and since there would be less then a child allowed to each person alive, the population would have to decrease. This is maybe a better situation than we observe in China where there is a strict limit to the number of children any couple may have. Those who can afford to have children may have as many as they like, and those who can't afford to may make some money off their child-right.
The most obvious problem with this idea is enforcement. Since many people, especially in the lower classes, are promiscuous, there may end up being extra children unintentionally, even after these lower class people have sold their child-rights. Humans will still mate even if they aren't supposed to and so we realize this is a very idealistic sort of thing. And even if contraception is used, there will be mistakes. There would have to be abortions allowed and who knows what governments would decide to do with children who are born without the parents having a child-right.
So I guess I would end up being against this idea if any politician tried to push it in my lifetime. But if we were forced into a population control measure by our government, I think I would like this program the best.