low battery life

I received my Ipod 6 months ago and after a month, the bluetooth failed. The local Apple Store replaced it. The second unit had a low battery life (about 15-16 hours). I contacted Apple again and they sent it to their test lab to check it out. When I got it back, the battery life was up to 28 hours and was in spec. Apple advertised the battery life at 40 hours.

Now after about 4 months, the battery life is 16-17 hours.

How can this Ipod be fixed so the battery life is up to specs? Or do I have another defective Ipod?

BTW: The Original Ipod had a battery life of 33-35 hours.