I guess one of mine would be recycling. Not just your household waste, but using second hand items where possible (clothing, baby furniture) in a pay it forward style system. Babies grow out of stuff so quickly, if you pass on everything in good condition, you'll generally find what goes around comes around should you ever need stuff again.
I followed a blog once from a lady who was not buying anything new for her baby (only the car seat I think) and it was really interesting. A lot of people turn their nose up at second hand things. Yet so much of the stuff we end up with we don't even use! If you put every item in a pile from the day you had your baby until the day they turned 2, it would be a huge pile! By using that stuff twice or more, you are reducing the number of those piles, reducing carbon, saving trees etc.
What about you?