I just have a flexible back. I don't purposely train for flexibility or the arch.. I have such an epic arch just to get maximum tightness.. If I had a choice I'd probably choose to not have to arch as much, its really not all that comfortable and always generates comments on my arch rather than my lift.. Why are you on such a mission to increase yours? For decreased ROM?.. I'd just work on increasing bench strength first?
Edit - also I have a big butt that makes it look a lot more arched