The IPod touch comes with a smaller battery than the IPhone to begin with. You must take certain steps to get the most out of your battery.
Here's what I do. Keep your screen brightness at 25-50%. Always keep Bluetooth off. When not using wifi, go to airplane mode. In settings, Turn off app refresh. Turn off spotlight search. Turn off location services, these are battery killers when on.
When you are not using your IPod touch. Turn it off, Not sleep mode, totally off. Like all lithium batteries if you get it below freezing in winter the battery will lose power quickly. It's only a temporary problem until it warms up.
When the battery is down to 20%, you can still go another half hour, before it turns off. It's good to run it down once a month to keep the cycle limber. Also it's ok to use it while plugged in, but don't just leave it plugged in all the time.