The pic on the left shows the starting position for a dumbbell row. The lower back is slightly arched, the head is up and looking forward, and the shoulders are not allowed to hunch forward. As I like to say, if you don't feel like the entire gym is staring at your ass, you're doing this one wrong. You can also straddle the bench and put one hand on it, instead of a hand and a knee. As the dumbbell comes up, think about pulling it towards the hip, not towards the ribcage.
Elbows stay close to the body through the movement.
Above all, don't allow yourself to twist to force the weight up. For back exercises in particular, it's better to go with a lower weight that can be moved with precise control. In the pic on the right, I am looking over my shoulder as I complete the rep. This isn't necessary to do, but apocryphal knowledge has it that looking over your shoulder helps prevent you from twisting the torso. Frankly, I think strict attention to not twisting and a slow controlled rep makes more of a difference, since I can still twist plenty in that position.