Enqueuing jQuery in plug-ins

First of all, if your code (or the theme’s code) is unregistering jQuery and re-registering it from another location (Google), you need to add this somewhere:

jQuery.noConflict();

This makes jQuery cooperate with other scripts (i.e. Prototype) that try to define the $ variable globally. WordPress has this line bundled into their version, Google does not.

If you need to enqueue jQuery and a bunch of jQuery UI stuff, you’ll need to do that.

function my_enqueue() {
    wp_enqueue_script('jquery');
    wp_enqueue_script("jquery-ui-sortable");
    wp_enqueue_script("jquery-ui-draggable");
    wp_enqueue_script("jquery-ui-droppable"); 
}
add_action( 'wp_enqueue_scripts', 'my_enqueue' );

This will hook your enqueues to the appropriate action that will output them in the header. I’m assuming that the jQuery UI scripts are already registered within WordPress.