Are you cruising or travelling out of the country and plan only to use WIFI. On my recent trip, I found that despite the cost of WIFI on the ship ($30/day for 2 devices), it was worth it so that we could iMessage each other during the day.
Initially, I simply turned cellular roaming and data off. I found that each day, my battery barely made it through. I was skimping on usage at the end of the day to make it last. It seems that my iPhone was working hard all day to find a non-roaming cellular signal.
The right way to do this is to go to settings and turn on Airplane mode. This turns off Cellular, WIFI and Bluetooth. Then individually turn on WIFI and, if you need it, Bluetooth. Now your iOS device will work well all day with plenty of battery life.
When you return to the US, simply turn Airplane mode off.