One option for people with some money to spare, but not enough to personally buy thousands of acres, is to donate to a conservation land trust, which pools money to buy up land and preserve it. These are usually local or state/regional organizations, such as the Texas Land Conservancy [1] or the John Muir Land Trust [2]. There's a directory of them here: http://findalandtrust.org/

[1] http://www.texaslandconservancy.org/

[2] http://www.jmlt.org/