Regular use of leather should break it in pretty quickly. A leather conditioner or wax can also speed that up. Another trick would be ground up coffee with a minimal amount of water to help break it in.
Essentially, whatever tricks work to break in leather.
Counterintuitively, a light coat of Kiwi shoe polish on that black case will act to make it a bit more grippy in-hand. I think it's because the wax in the polish overcoats the lanolin-rich fresh leather.
Or just wipe the case gently with a damp cloth- this will make the leather a lot more grippy.