The only secret i have for the arch, btb, is using a foam roller. Put it under your back and try to make your traps and butt touch the floor at the same time... in the past i have benched with weight plates shoved under my low back but thats kind of dangerous. You can bench with the foam roller under your back though. Other than that, its just working on it. Its just like anything else, if you dont deliberately try to improve it, its probably not going to get any better...
I know not everybody reads my log, so i may make a new thread anyway sometime soon.