Still, this might be useful.

I heard about this on the Internet and have tried it for two days (with some cheating; I admit):

Turn YouTube off. It's not enough to not use it because it still runs in the background. Make sure it's turned off. Make sure the icon is not in color. It needs to be grey when it's on one of your phone's operational screens.

The battery (I haven't timed it) seems to last about 1 1/2 times longer. Yes, I estimate it at 150% as long, give or take.

I highly recommend this if you need to rely on your phone as... a phone, send and receive texts, and have Internet access it you need it.