For best performance, it's a good idea to keep your soft shell clean, and its DWR (durable water repellant) finish is freshly charged. This means keeping oil and grease away from the garment, washing it when it gets dirty, and reapplying its DWR finish.
Soft shells can be machine-washed in warm or cold (not hot) water, using regular powdered laundry detergent. Wash in a front-loading washing machine only, and do not use Woolite, other cleaners with lanolin (or other oils), or fabric softener. Put the garment through at least two rinse cycles, as the detergent can interact with the DWR finish and needs to be fully rinsed out of the material.
After your soft shell has been washed, reapply the DWR finish while the garment is still damp. The heat of the dryer will help set the DWR finish into the fabric. Tumble dry on low heat, or line dry.
Mountain Hardwear recommend you re-treat your soft shell at the beginning of your wet weather season; however, if you are washing your soft shell often, or if your soft shell is frequently in contact with abrasive surfaces, they recommend re-treating the DWR finish more frequently. For best results, use spray-on, rather than wash-in products. They recommend ReviveX. Follow the manufacturer's instructions for application.