How do i not blow through my data cap?

I've been on the iPhone 5 for more than a month now, and with very average use, im blowing through 500mb a day almost, to the point that about 2 weeks in i have to start ultra-rationing my data usage. Even though i switch to wifi when i'm home, but if i watch a 5 minute youtube that eats about 100mb from my data.

This was never an issue before because the youtube app on android defaults to low-res when on cellular data, unless you specifically click on HD. But the iPhone youtube app doesnt have an option to adjust bitrate, it automatically decides to load the HD version when i'm on LTE, since the connection is better.

Any way around this? How do I get it to load non-HD videos by default?

EDIT: Just to make clear, i'm on a 4gb LTE data plan, previously i used to use a 2gb unlimited T-mobile plan, and very rarely went over 2gb, despite some very heavy usage including app updates and music streaming every now and then. My browsing behavior is unchanged, Now i've even set apps to not update, and i only update over wifi. Even a 20 minute facetime session uses about 200mb... :/ so i've even set that to "wifi only". Data is just bleeding, and this is after the verizon update that was causing 1gb a day data bleed even when on wifi. that is fixed now.