Work your hip and hamstring flexibility and mobility too. I though my lower back was tight a while ago, but it loosened up from improving hip and ham flexibility.
For back and hip flexors specifically, I like to have my heels 6 or so inches from a wall and lean back and put my hands up and back and walk down the wall as far as I can and hold the position for time. Also just standing up an bending back far. I actually do that latter after DLs or GMs about 10 times so the fluid in my disks remains where it should be.
For bench, you can try using pvc pipe under your back when you warm up. Never tried it myself, but I've heard of people having good luck. I've seen boards used too. Just be careful not to do too much too soon because it is possible to tweak your back from benching.