Many aspects of green building are so much easier and less expensive when they're built into a home. For lots of us, this isn't a choice - we have an older home, and like both our home and our neighborhood. What can we do?
When your hot water heater gives out (and they all do) - try tankless water heaters, which are so prevalent in Europe.
When you buy light bulbs, buy compact fluorescent bulbs, and replace all of the incandescent bulbs, one at a time as they burn out.
When you need a new roof, look into a metal roof. These help reflect heat away from the house, and keep the attic cooler, which means lower energy costs. Whether it's a Key West cottage or a new luxury home in stucco, metal roofs can be very attractive as well.
The point is - whenever you have to replace something, from windows to carpet to light bulbs, look into energy saving options and recycled or renewable materials, Every little bit helps.

A couple others that we tell our home buyers:
Green building is big right now ... but Green homeownership is more important for the long term.
Great post Sharon.
Last year our heat pump went kaput. Had a state of the art one installed. Operating costs went down significantly. Would not have replaced it solely for that reason.
John - 13 years on a light bulb is certainly a testimonial!
Ed - with older homes I think it's logical to wait until something needs to be replaced.
Joan - you're right. Steve added some ideas, and I'm sure most of us know at least one way to save.
You can also install a rainwater collection system and use the collected water for a lot of things, including watering the lawn and any animals and sometimes even washing your clothes (much softer water). There are even those in town in Austin these days. Quite trendy.