Well you have a few options really: you can go for an external power pack or a battery case.
I recently wrote about the Limefuel extended battery case for the Nexus 5 to give you an example of the second: www.androidpit.com/limefuel-battery-case-for-the-nexus-5
As for power packs, there's so many out there it's not funny. You can pick up 25,000 mAh battery packs if you want to! Basically, you just plug your device in with a USB cable and it'll quickly charge your battery up while you're on the move. The thing is, the bigger capacity ones are quite big and heavy. Smaller ones (around 3,000 mAh) are pretty portable and light, and I have one in a jacket pocket when I expect to be out and about a lot.
But as I said in the Limefuel review, I can't really imagine suing a battery case as a permanent accessory. it's just too bulky for me (although I usually don't use cases at all, so...)