My small motorbike, loaded with my luggage, ready to board the ferryboat going to Makassar - Surabaya (Java Island, Indonesia)
To protect my backpack from the rain and make it inconspicuous, I always put it in rice bags. I used rice bags also to make makeshift side panniers, where I put tools and extra gas.
Locals are able to fit even more luggage on those small motorbikes: www.flickr.com/photos/loupiote/52737958829.