Modify POST vars before post, using jQuery

Use the submit() function on the form to create a callback. If the function returns true, the form will be submitted; if false, the form will not post.

$('#theForm').submit(function() {
    $("#field1").val(newValue1);
    $("#field2").val(newValue2);
    $(this).append(newFormField);
    return true;
});

etc.