Turn denim inside out before tossing it into the wash. This will help protect the color of the fabric.
Wash denim in cold water.
Add 3/4 c. of white vinegar or 1/2 c. of salt to the rinse cycle in lieu of fabric softener.
Place a few clean, new tennis balls into the dryer with your denim while the garments dry. The balls will agitate the fabric and make it softer.
Soak the denim overnight in two caps full of fabric softener. Wash and dry as normal the next day.
Repeat all or some of these steps if the denim isn't soft enough. It may take more than one cycle to make the denim as soft as you want it to be.