Future Social Solutions
Mega-structures/Cities
The solution to cramped spaces is to create more housing as well as making it affordable to the population. They could work very similarly to apartments with multiple families living on each floor just like in the Judge Dredd comic book series. This will cost the Earth some resources but well worth the investment if the Earth begins to become even more populated with humans.
http://megacities.nl/
http://megacities.nl/
Terraform
This solution is a little more futuristic. The technology would need to be developed such as creating an atmosphere, as wells as water, and plants and animals for food. This process of turning another celestial body into a faux Earth could be preformed on either a moon or planet just like in Red Mars. The most logical celestial body to terraform would be Mars seeing as how it is essentially the same size as Earth with the closest known gravity. Adjustments to living on this planet would take some time for the colonists to get used to but would certainly alleviate some of the pressure that the overpopulation was impressing upon the Earth's environment and resources.
http://www.howstuffworks.com/terraforming.htm
http://www.howstuffworks.com/terraforming.htm