The planet is far from overpopulated, it's just being overworked for resources. If we were to stand every human shoulder-to-shoulder, it'd barely fill up downtown Los Angeles. We do consume a large amount of resources, though. At some point we'd have to colonize the moon and maybe Mars, to spread out a bit.
Turning towards renewable energy sources like solar, wind, and water would go a long way to protect this planet. Strip mining and tree chopping would have to be outlawed, at some point. Farming and fishing would have to be encouraged but controlled. It'd be a huge administrative task for the entire planet.