I got up last year to do the fascia and soffit and just used warm water detergent,and one of those large sponges,rinsed them with the garden hose and put thompsons pvc cleaner on them, they came up really well. not something i would normally do but i was getting he house painted and i reckoned they would look pretty rubbish if i didnt, bit of time and effort but it was welll worth it,i had tried the power hose before but it didnt get them anywhere as good as they look now.