You're talking about exercises or tricks to become a shower rather than a grower by nature. The only way to do that is to get your dick used to it. You can try wrapping, which is great for it if you're good at it. I like to do lazy ass stretches where you literally pull your dick down and sit on it. You can also try pulling it between your legs and holding it with your legs together while you sleep. Also just stretching it out whenever you hit the bathroom can help.
Your dick just has to get used to it, which will take a while, but a big flaccid hang is definitely worth what little effort it would take.