Practice, stretching, and maybe some foam rolling should help.
There was a video I was going to post that describes set up and ways to improve your arch, but it seems to have disappeared on youtube
Be sure to warm up and stretch, especially your hip flexors.
People have used foam rollers and PVC pipes as a tool to arch over, stretch and learn to keep their arch up while benching. I don’t know exactly how extreme you’re trying to get your arch, but you kind of need to have a decent arch before you can do some of that work.
There seems to be 2 basic setups for arched benching and it seems that one is most popular among girls and smaller guys and the other is what the big guys tend to do.
This is the type of setup that I find produces the best arch; feet relatively close together, tucked under you. Setup the legs then lower your upper back onto the banch. Hannah Johnson’s setup requires a bit more skill and flexibility than other, similar ones, but is still a good example.
The other way more or less involves laying on the bench, arching, and setting your feet up. At least, thats how it seems. I don’t really have any experience with this setup.