Well, I use an app called battery saver (root). You'll find it in Play Store. It's for root users, so you need to be rooted.
What it does: Battery Saver (root) will change the system ROM configuration for better battery life. It won't cut off WiFi, mobile data etc while the phone is in sleep like other apps does.
Try it for some days, it should help you like it did for me
Sent from my GT-I9300 using xda app-developers app
Current Smartphone: Samsung Galaxy S III
My Smartphone history: Sony Ericsson Xperia x10 Mini Pro -> Sony Ericsson Xperia Arc S -> Samsung Galaxy S III