To keep the waterproof coating in good condition we recommend that the coat is only put through the washing machine when absolutely necessary. After muddy walks we tend to rinse our coats off with the hose as soon as we're back, shake or towel off excess water, and then hang to dry. This way the coat doesn't have to go through the washing machine too often.
When you do want to wash the coat thoroughly, it is best to choose a cool setting (around 30 degrees) and use mild detergent. Please ensure you do not use fabric conditioner, or detergent containing any fabric conditioner. If you have a hand wash cycle, use that.
If you'd rather just stick it in the washing machine regularly, another option is to buy a waterproofing spray online - spraying the coat every few months should repair/replace any damaged parts of the waterproof coating.