Population control on a global scale is not going to happen, I think. People are just too different and too incapable of understanding each other.
The problem with overpopulation is really condensed to third world countries, whereas economic growth often outstrips population growth in first world countries. In fact it is constantly argued in economic history and the geography of economics that the persistent high population growth in these third world countries is what has kept them as third world countries (because the economy has been unable to create jobs, wealth or life quality for everyone new).
In a free, democratic society population control is unlikely to happen on a national scale. Generally, people do not consider themselves a net deficit for the environment (but most are).
I think if one wishes to limit population, one must look to the fundamental reasons for having kids. One of the main reasons for having kids in a third world country is to ensure your own economic safety. These countries lack social security, and they lack any form of government pension for when a person gets too old to work. And, in fact, it is often not possible to save up for a pension on your own salary. And because of the population growth which keeps salaries near the absolute bottom, the next generation is forced into the same situation.
Attitudes and religion are also important, although very little can be done about those.
Available contraceptives is another method of voluntarily limiting population growth. A government seeking to limit the amount of kids could support such initiatives (and, similarly, not pay people to have kids like some countries do).
If you're worried about overpopulation, I think it's important not to help people who are starving in areas of constant growth. As cruel as it may sound, and this is coming from someone who has lived through rough times, if a population lives over its production means, it should be forced to shrink or consume less.
Edit: If the reins were free, it might be a good idea to outlaw religion and porn. Those are probably major contributors to pregnancies in first world countries. But honestly, at that point one might as well force sterilization on people with x amount of kids.
And of course, there's also the possibility of covertly spreading diseases to countries that cannot afford vaccinations.