What practical changes to our lifestyles can make a difference to sustainability?

Until a couple of hundred years back, we all lived lives that were in harmony with nature, and everyone – including the earth – was happy. But then our lives and lifestyles changed, and today we find that billions of people lead lifestyles that are poorly aligned with our natural environment. Perhaps we cannot wean ourselves completely away from our current lifestyles, but surely there are simple but practical concepts we can adopt for a start? What could be these practical concepts and ideas that can make modern lifestyles more environment-friendly and sustainable?