Use this method:-
(http://www.youtube.com/watch?v=k6cPLBonMXA)
Or, put them under the Sun to oxidise any Yellow.. (But mind you that be careful to NOT burn anything!)
But NOT to Do thing like this: http://www.hermes.net.au/bayling/repair.html
This guy is just crazy! He executed a great glass! Yellowing is Not Guilty!! =_="