Cellphone rental is no longer as necessary as it used to be. I think that both ATT and Verizon offer $10/day plans, which will only apply on the days which you power-up your phone and use it. I have different needs, and use the older ATT Passport program, which fits better with my extended stays (looking at 28 days on my upcoming inter-island trip).

Consider this - if there was a good profit margin in providing a cellphone rental service then someone would. And they would advertise it. Can you hear the Silence?

For the $10/day plans, turn your phone off if you don't need it. Keep it in Airplane Mode and use Wi-Fi if it is available, as Wi-Fi more widely is. Turn on Cellular Roaming only if you need to.