One of the easiest and most effective ways to soften stiff fabric is to place the dry fabric in a clothes dryer on the "tumble" setting. Alternatively, place the fabric in a washing machine and add a homemade softening compound made from hair conditioner, apple cider vinegar and water. Fill the machine halfway with warm water, add a cup of softener and soak the fabric overnight.
The homemade fabric softener technique works with most fabrics. To protect delicate fabrics labeled "hand wash only," soften them in a clean plastic tub filled with cool water and several tablespoons of fabric softener. After softening, rinse the garment with a fresh change of cool water.
According to About.com expert Mary Marlowe Leverette, baking soda helps soften fabrics, particularly cotton sheets. It also improves the efficacy of bleach, boosts detergent action and removes lingering foul odors. Fabrics with severe odors benefit from an overnight pre-wash treatment of baking soda mixed with plenty of cool water.
To increase the softening power of baking soda, add a cup of white or apple cider vinegar to a solution of water and baking soda. This creates an energetic foaming reaction that loosens fabric fibers.
In an article for LifeHacker.com, columnist Kevin Purdy shares his technique for softening starched bed linens. Purdy recommends placing them in a washing machine full of hot water and running the machine without adding detergent or fabric softener.