Be kind and patient about it. That's what I would want if it was my kid crying his lungs out on a flight. The parents must be just as helpless and frustrated. I usually just crank the volume up on my in flight entertainment system and make do with the situation. Once I was sitting next to a mother and crying child (she was travelling with her baby alone) and I helped distract the baby when mommy wanted to eat and held the baby if she needed to go to the bathroom. I always tell myself to treat others how I'd like to be treated one day :)
All you can do is put cotton balls in your ears or put the volume of your iPod or inflight music system. The babies get cranky during flights and they too do not like being on the longer flights. I guess, you no choice but be patient and bear it. No baby cries out of choice and the ones that do, parents know how to set them right. :)