I personnally don't worry about this kind of thing. I just stare at the cake with so much envy that I forget to blink, thinking: "hurry hurry hurry hurry hurry".

Smile and look vaguely embarrassed.

One important rule is never to look directly at the people who are singing - or at least not look at any one person for more than a second or two - because they're probably as embarrassed as you are. Boubouillehou's idea of staring at the cake is a good alternative.

It can be embarrassing at times.  I keep a low profile on a lot of things including birthdays and hope nobody will remember but it is next to impossible especially at work where everybody knows you and you hold a high position in the organization.   

Hello 

You have asked a very tough question. I try to smile normally but it almost always turns into awkwardness.