One of the biggest reasons we have urban decay is the simple fact that it's usually more profitable for developers to start with undeveloped land than to renovate or re-purpose existing structures.
We could level the playing field by using the tax code. The result would yield huge economic, ecological and sociological benefits. There already are some elements of this in the tax code, but more is needed.
I would even go so far as to say, we would benefit if the tax code made it significantly more profitable for developers to renovate and reinvest in existing areas than to bulldoze new undeveloped land.
One of the biggest reasons people give for abandoning their cities and moving to bedroom communities is lower taxes. That's pretty easy to fix. There should be a tax on the people living in these bedroom communities and let the proceeds go to the city they abandoned but still feed off of economically.