You posted this in the "Remodel & Renovate in Seattle" area, but don't mention homes around here... so I'll talk about our market - Seattle is considered a very "green-conscious" area.
I work with buyers every day and "green" is one of the questions I ask in our interview process.
As much as I'd love to say that green features make a difference, they aren't going to make the impact on a dollar-for-dollar basis. "Green" tends to be a label that people associate with charging more for a home for little benefit and in these challenging times, price is king.
That's not to say that there isn't a market. There is a niche market for green-conscious consumers who are willing to pay more for these options, but in the Seattle area, it's still a "relatively" small group of people.
As far as sustaining your homes' values...
Generally, your biggest bang for the buck is to keep "deferred maintenance" to a minimum. That will also help the environment. Keep insulation tight and dry. Upgrade older homes to double-paned windows. Keep water away from the wood siding. Keep gutters clean and draining right. Do regular pest inspections to ensure nothing's eating your house away!
Interior: watch for trends in finishes and appliances. Granite and stainless (or brushes metal) appliances have been the big differentiator in many home sales for recent . Staging is crucial at selling time.
Exterior: clean or fresh paint and a clean or new roof make a huge difference to a home. A neatly manicured front yard will sell a home in a moment.
I hope this helps!
At this point, "Green" features will serve to make your home more attractive to some buyers but for the most part will not greatly impact the value of your home. A home constructed as a totally "Green" home is another matter and would have value.