The best lower back exercise(this is also a great hamstring stretch) is called the russian deadlift, we used to do it during track season while we still trained for football.
Get yourself a barbell and probably 25s on each side(the weight isn't as important as this is a stretch), get a box or something solid to stand on, like 6 inches off the ground, and just slowly, straight backed and straight legged, touch your toes with the weight in your hand, then go back up. The longer you take/hold it, the better.