Having covered a couple of shows, I've always found even the best mobile tends to flag, and have been using an external battery charger for more than a year. These are particularly great if you're going hiking or camping and use GPS a lot.
The TP-Link TL-PB10400 offers 10400mAh of energy. That should be able to provide at least 3 full charges for virtually any device. TP-Link claims that it has 90% energy efficiency rate, compared to most others which hover around 80%. This means that more of the stored energy is transferred to your gadget.
It's a very simple looking device roughly measuring 9cm x 4cm x 4cm and weighing 249g. It feels solid, and comes with one USB-micro USB cable (Apple device owners will have to supply their own cable), and a small pouch. It has two sockets on the top (1 Amp and 2 Amp), as well as one button which switches on the small flashlight, or tells you how much charge remains. using the four tiny LED lights. It's charged via socket on the side.
You can charge two devices from it simultaneously, and it has circuit breakers inside which prevent you overcharging and damaging your device. That's about it. It works as advertised, although it won't be replacing my current charger as I find the shape awkward as it doesn't fit in my pocket nicely, and it's a bit heavier. At roughly £20 though, it is also cheaper than what I use and therefore recommend it for anyone looking for an affordable charger which could do multiple charges of even the most thirsty phone.