Set select to post-value else use given value

[eluser]Grind[/eluser]
Of a <select> menu, how can display the menu-item that was selected according to the post value and - if the post value isn't set - to a given value? Just like you can do with text fields:

Code:

<input type="text" name="username" value="<?=set_value('username', $userinfo['username'])?>" />

Is my question clear enough?

Thanks in advance.