I do a few things... I have an iPhone and pack a charger with me.
I always make sure I am at 100% in the resort and then I put my phone on "airplane" mode - this means I can't send/receive messages OR use the internet OR My Disney Experience. However, when the phone is on airplane mode it isn't constantly searching for an internet or cell connection.
I turn my "brightness" wayyyyyyyy down. This helps conserve battery life, as ewll.
You can still use the camera settings during this time, but you can't do any of the texting/internet. It works for me!
Then, at lunch or during any down time - especially in the MK at Rapunzel's restrooms (a.k.a Tangled Tinkle Towers) - I charge when I can. My phone has NEVER died on me in the parks using these tips! I just have to cut back on my regular facebook /
instagram / messaging / twitter tendencies!