I wrote a bit about this right as iOS 7 was coming out, so now that iOS 8 has just been announced, I wanted to link to this excellent, exhaustive blog post by Scotty Loveless, The Ultimate Guide to Solving iPhone Battery Drain.
The TL;DR: Turn off background fetch on any apps you don’t want to prefetch content for you (Facebook is apparently a nasty culprit on this), and quitting your running apps is actually *worse* for battery life.