Use :

static $default_sort = 'LastName ASC';

in your DataObject.

Legend!! I spent a few hours trying to figure that out yesterday and with your help done in under an hour - thanks heaps :)

Code below:

<?php

class Staff extends DataObject {

static $default_sort = 'LastName ASC';

static $db = array(
'FirstName' => 'Text',
'LastName' => 'Text',
'Email' => 'Text',
'Location' => "Enum('Wellington,Auckland,Christchurch,London','Wellington')",
'JobTitle' => 'Text',
'JobDescription' => 'Text',
);

static $has_one = array(
'Page' => 'StaffPage',
'StaffProfileImage' => 'CustomImage',

);
static $fields = array(
'FirstName' => 'FirstName',
'LastName' => 'LastName',
'Location' => 'Location',
'Email' => 'Email',
'JobTitle' => 'JobTitle',
'JobDescription' => 'JobDescription',
);

public function getCMSFields_forPopup() {
$fields = new FieldSet();

$fields->push(new TextField('FirstName', 'First name'));
$fields->push(new TextField('LastName', 'Last name'));
$fields->push(new DropDownField('Location', 'Location', singleton('Staff')->dbObject('Location')->enumValues()));
$fields->push(new TextField('Email'));
$fields->push(new TextField('JobTitle', 'Job Title'));
$fields->push(new TextareaField('JobDescription', 'Job Description - write a couple of paragraphs to describe your role at CDP.'));
$fields->push(new ImageField('StaffProfileImage', 'Staff profile image'));

return $fields;
}

}
?>