The same principles low density cities were build. By doing whatever is possible to make city centers more attractive. Stop any subsidies for private car ownership. Build functioning public transportation networks in city centers. Make driving there less attractive, too expensive and simply unnecessary. Locate businesses back in city centers instead of building them in the middle of nowhere, in giant ugly malls. It's a long process. It's an expensive process. But so was the construction of hellish asphalt deserts such as the city of Houston, which is no more than a miscarriage of city developing. And obviously you start with the most doable stuff and utilize the resources already at your disposal. Better public transportation? You already have a shitload of streets. Buy busses, make them cheap, clean and reliable and go from there. And houses can be torn down. So can malls. Renaturation is possible and has been done before.
American cities are a suburban nightmare. And they are so by choice. And these choices were also incredibly expensive, difficult, took decades of (idiotic) planning and were no less of a monstrous task to enact, than reversing this wrong course could be.
I absolutely know that I'm demanding an awful lot and that it is in no way realistic that we will see it happen. But I'm also convinced that it is necessary, doable and the right thing to do. And personally that's more than enough reason to do it, for me.
Oh, there's also nothing futuristic about it. We already have cities like this and there are more and more going this way.