How to stop jeans going hard after washing them?

I don't think there is a way. But they soften up reasonably quickly when you wear them..

You should dry them outside on a clothes line, preferably on a windy warm day... you will notice a BIG difference if you dry them this way

use a water softener

There is no way to stop this.  Denim is a thick fabric and it takes time for it to soften.

I always iron the jeans after I wash so it retains the shape and also becomes softer. Use the steamer setting in your iron and keep blasting it as you keep ironing. That will soften it. 

5 minutes in a tumble dryer will soften hard dry jeans , old army trick . likes the mrs sweating trying to iron my jeans ..lol it works every time ..lol . try it nice warm jeans before you iron them , but dont tell mom , more fun seeing her strain ...

Any fabric dried by hanging is going to get stiff.

Just shake them til they soften up.  Like REALLY shake them, make them snap several times and they'll soften right up.

I NEVER use fabric softener, it leaves stuff on the fabric and makes it reek.  And it won't solve this problem for you.

Other than using a dryer, probably not.   Smack them on the floor a few times to soften them up.

If you have everyday (inexpensive) denim and don't want to pay a dry cleaner, then wash them inside out in cold water and add a full cup of white vinegar to the washing machine. The vinegar with hold the color dye in the fabric and will act like a natural fabric softener.

If you do not dry them in a dryer, then they are going to dry stiff.