When I was younger overpopulation was my personal soapbox. I would bore my friends repeatedly with my rants pertaining to it.
While I still recognize it as a problem, I have also come to a similar conclusion as some who mentioned this at the beginning of the thread: the majority of our problems seem to stem more from a messy uneven distribution of people, than the actual number of us. Also handy to this opinion is the fact that I don't live in a large city, and I attribute a fair amount of insanity to those who would willing choose to do so.
I have always maintained that humanity must approach the problem of overpopulation in the most humane way possible. Unfortunately, the only method I've ever been able to come up with that fulfils this notion is for people to simply choose not to breed; an unpopular choice, obviously.
Outside of a worldview and recognition of the problem however, citizens of America at least really have little reason to stop breeding. Society encourages it, as the churches want larger congregations (and offerings), the military wants more soldiers, the government wants more taxpayers, and the commercial complex wants more consumers. Our society goes so far as to grant monies to people who have little to no money simply to bear more children. Sure, it's rationalized as providing a possible better future for the child, but ultimately there are few restrictions on what that money gets spent on.
So many people grow up in America thinking that "getting married and having kids" is simply What You Do (tm). It's expected, and those who don't breed are viewed in a suspicious light. Padawan mentioned that he is viewed by breeding friends of his as being selfish, when in fact, it is not uncommon for (especially women, but certainly men as well) to desire a large and loving family because they don't feel they're loved without children and usually also have a warped sense that their offspring can be forced to love them. I generally consider breeders as much more selfish than those who choose not to have children (unfairly, because of the warped examples).
I believe that, before any useful and measurable decrease in population will ever take place in America, an extreme change of view and action pertaining to many different aspects of childbearing would need to take place. Social support for and of childbearing would need to be eliminated. Childbearing as a goal in life would need to be disregarded entirely. Education about the bindbending expense and effort childbearing and raising requires would need to take place at a level equal to subjects we consider essential, like reading and basic math. Ultimately, I think even the "sanctity of life" would need to be diminished in order to turn around humanity's obsession with excessive breeding. Religious suggestion of a divine decree to "go forth and prosper" (with an implied meaning of reproduction) would need to be eliminated. Institutions such as Mother's Day and Fathers Day would need to either be met with equal attention by something like Non-Breeder's Day or be eliminated. Celebration of being a single child could also help in this respect.
Like Padawan, I have decided never to have children. Thankfully, I've found someone to share my life with who has made the same decision, though one or two past relationships have ended because of my decision in this regard.
If we were willing to compromise our humanity to address overpopulation, I belive one of the best ways would be randomized (lottery) sterilization. Although it could begin simply by offering free and safe sterilization to those who want it. I'd be up for that. Forced sterilization following a parent's first child's birth would probably be fairly successful.
We certainly could try more social reforms like China's birthing policy, but I don't think it would ever fly in a society like America. I think many deaths would result in the wake of such things... which, of course, would further the aim of reducing the number of people on the planet, but in a way unintended.
Ignoring medical advances does cause a fairly serious problem. Much like the addage, "That which is learned cannot be unlearned," I believe it would be nearly impossible for society to stop utilizing the medicines and procedures we currently have available to us. I can admit I personally would not be able to do this, even keeping overpopulation in mind. However, I do believe society should embrace assisted suicide and abortion.
War, of course is something of a resolution, though it also comes with a sense of the people that the casualties must be replaced.
Should we depopulate though? Do we need to? I believe we should, in the interests of doing as little damage to the earth's resources as possible. I don't necessarily believe that we need
to, as I, like many here, believe that when humanity grows enough it will inevitably be diminished by plague, war, or some other force. But I also believe that the majority of the survivors would simply see this as a situation in which they're practically duty bound to breed like mad to replace those who have died - so the problem wouldn't be entirely solved.
And ultimately, aside from volunteer abstension from breeding, I doubt many societies would accept any of the other possible methods of intentional depopulation.
(I apologize with any inconsistencies in my train of thought in this matter. I have several related opinions, and to date, have not codified them into a paper of any sort... something I aught to do eventually)