How do I improve my iphone4S's battery life?

I have never had any problems with my Iphone4S before (its about 6 months old) but in the last few days the battery life has hugely decreased. If I fully charge it, it has no battery left again four/five hours later, even when I'm not using it that much. I have tried turning down the brightness on my phone, closing all apps running in the background, and turning off bluetooth etc to save battery but it is still terrible. Please help!