Originally posted by Oso
my best answer is to pull the cork on the space program and open it up to commercial interests and get some of us of this friggin planet.
I've always been highly dubious of the relative rewards of the space program. An ocean program would yield significant rewards far sooner, be far more economically efficient, and would translate directly into a more efficient space program upon it's culmination. Aiming for Mars when we haven't dipped our foot in the majority of our own planet seems a little misguided.

Another excellent answer would be to address population distribution rather than population itself. The earth as a whole, even restricting ourselves to habitable landmass, is far from being overpopulated - it's just that the population we have is poorly distributed. Again, rather than aiming for Mars, we should aim for information, energy, and good distribution to allow for a more distributed mode of living.