not many muscles back there dude but those are the ones you should have been working all the time... bad backs are way to common in men these days... heres a few things i do
full bend over... take some dumbells, and hold them down in front of you around your hips keeping your arms straight... just bend over til you cant go further then come back up to your hips... simple as that... thatll work the main back muscles...
second, side twists... take a weight, anything you can hold and do a wall sit on the wall or something... just take the weight hold it with your arms straight and do full twists, usually best with a partner... back to back passing to each other
lastly, backwards crunches... get something that allows you to lay flat and hold your legs down... and just do exactly what i said, this also works great with a weight, just hold a weight to your chest and lift if with your back...