Back extensions are one of the most basic, yet effective exercises that you can use to build your spinal erectors (the muscles responsible for the "back valley"). Almost every gym has a 45-degree back extension machine. Brace your feet on the platform then fold your torso over the pad with your lower back flat. From there, don't just throw your upper body back wildly; think about squeezing your lower back all the way up. Hold at the top for 1-2 seconds, and then slowly lower yourself back to the starting position. Four sets of 12-15 reps are great.
Barbell Good Mornings
The good morning is traditionally used in the power-lifting world as an accessory movement for the squat. However, the strength it adds to the erectors will carry over into carving out your "back valley." With this movement the weight is not as important as form. Let me reiterate—form is key! Start standing with the bar on your back as if you were about to perform a squat. From there, push your butt back. Over exaggerate this feeling. The good morning is about horizontal hip movement. Your upper body should come down to a point just above parallel (depending on your flexibility) before returning to the top. Three sets of 10-12 controlled reps are perfect.
Functionally speaking the "back valley" is one of the most important parts of your body. But from a physique standpoint, it is one of the most impressive attributes you can possess. A stronger, better looking back is something we can all benefit from!