I only use Pastie for work, and so rarely have reason to share snippets publicly. During a design discussion, it’s easy to forget to tick the ‘private’ feature – hence this GM script, which does it completely automatically. I forget my head sometimes…

// ==UserScript==
// @name           Auto-private Pastie
// @namespace      PrivatePastie
// @include        http://www.pastie.org/
// @include        http://pastie.org/
// ==/UserScript==
(
    function() {
        window.addEventListener(
            'load',
            function() {
                var el = document.getElementById('paste_restricted');
                el.setAttribute('checked', true);
            },
            true
        );
    }
)();

Updated 15 Mar 2011: added second URL without ‘www’ alias.