In the book "schott's original miscellany", there is a graphic of which you are supposed to make a stamp. It says "VIP CUSTOMER UPGRADE WHERE POSSIBLE" I'm assuming its a fairly standard thing across the airlines.
I've not tried it myself, because i've not been out of Europe for 2 years, but will definatly try it when I go on a long haul.
if you want a copy, i've scanned it in. PM me your email address and i'll send it over.