Since jQuery Masonry repositions elements purely by positioning, it does not play well with UI Sortable. People have posted complex solutions to this problem, but this simpler solution worked for me:

1. Refresh masonry layout on Sortable’s start, change, and stop events
2. While dragging, remove from the dragged item the class used to indicate it’s a masonry item

var $c = $('#my_container');
$c.masonry({
    itemSelector: '.masonry-item'
});
$c.sortable({
    start: function (e, ui) {
        ui.item.removeClass('masonry-item');
        $c.masonry('reload');
    },
    change: function (e, ui) {
        $c.masonry('reload');
    },
    stop: function (e, ui) {
        ui.item.addClass('masonry-item');
        $c.masonry('reload');
    }
});