Simple. Don't have contribution agreements. Contributors maintain their copyright so as they can prevent relicensing. Mind, FSF requires copyright for their sub

Simple. Don't have contribution agreements. Contributors maintain their copyright so as they can prevent relicensing. Mind, FSF requires copyright for their submissions (I believe), but they're, arguably, a "good actor" in this space. But, if the code base becomes a patchwork of contributors, it can become difficult to relicense.