gistfile1.html

 <form id="example-form">
                <select id="exampleSelect"> 
                    <option selected>Primera opcion</option> 
                    <option>Segunda opcion</option> 
                    <option>Ttercera opcion</option> 
                    <option>Otra mas</option> 
                    <option>La ultima.</option>
                </select> 
                <div>

                </div>
                <input onclick="$('#exampleSelect option').remove()" value="Clear options" type="button"> 
                <input onclick="exampleAddOption1()" value="Add option" type="button"> 
                <input onclick="exampleAddOption2()" value="Replace options" type="button">
                <input onclick="$('#exampleSelect').html(exampleBackup)" value="Reset" type="reset"> 
            </form>
        </div>

        <script src="http://code.jquery.com/jquery-latest.js"></script>
        <script src="http://twitter.github.com/bootstrap/js/bootstrap.min.js"></script>

        <script language="Javascript">
            exampleBackup = $('#example').html();
            function exampleAddOption1() {
                var index = $('#exampleSelect option').length + 1;
                $('#exampleSelect').append('<option value="example' + index + '" selected="selected" Opcion #' + index + '</option>');

            }
            function exampleAddOption2() {
	
                var newOptions = {
                    'rojo' : 'Rojo',
                    'azul' : 'Azul',
                    'verde' : 'Verde',
                    'rosa' : 'Rosa'
                };
                var selectedOption = 'verde';
	
                var select = $('#exampleSelect');
                var options = select.attr('options');
                $('option', select).remove();
	
                for(var index in newOptions) {
                    options[options.length] = new Option(newOptions[index], index);
                }
                select.val(selectedOption);
	
            }
        </script>