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You can support the Rocky Mountain Elk Foundation and Backcountry Hunters and Anglers, both of which are heavily involved in issues of access for all types of backcountry users. RMEF especially specializes in purchasing land rights which unlock thousands and thousands of acres of public land that was previously inaccessible. I don't think any organizations are doing more to promote access on public lands. From their website: "Since 1984, RMEF has opened, secured or improved public access to more than 1.1 million acres of elk habitat across the country for hunters, hikers, anglers and other outdoor enthusiasts to enjoy."