A simple nav-bar locking jQuery script.

nav.js

/**
 * Navbar Locking
 */

$(function () {

    var $window = $(window),
        $mainNav = $('.main-navigation'), // nav wrapper element
	    stickyNavTop = $mainNav.offset().top;

    // A helper function to check whether nav should be fixed
    var stickyNav = function () {
        var scrollTop = $window.scrollTop();
        
        if ( scrollTop > stickyNavTop ) {
            if ( $('body').hasClass('logged-in') ) {
                $mainNav.addClass('under-admin-bar fixed-nav');
            } else {
                $mainNav.addClass('fixed-nav');
			}
		} else {
		    if ( $('body').hasClass('logged-in') ) {
				$mainNav.removeClass('under-admin-bar fixed-nav');
			} else {
				$mainNav.removeClass('fixed-nav');
			}
		}
	};

	// Initialize nav classes...
	stickyNav();

	// Then re-run on scroll
	$window.scroll(function () {
		stickyNav();
	});
});

style.css

.fixed-nav {
  position: fixed;
  width: 100%;
  left: 0;
  top: 0;
  z-index: 100;
}

@media only screen and (min-width: 600px) {
  .fixed-nav.under-admin-bar {
    top: 46px;
  }
}

@media only screen and (min-width: 768px) {
  .fixed-nav.under-admin-bar {
    top: 32px;
  }
}