It's a perennial problem, particularly with the high-power smartphones of today: they can do a lot but run out of grunt before the end of the day. It's okay if you can top up your phone in the afternoon that often that is not the case.
We have reviewed several alternatives, like fuel tanks and the like but the most obvious solution is something like the PowerSkin. It's a highly protective case with a built-in battery that in most situations will almost double the endurance of your phone.
Let me tell you why I like this case so much!