I just been to Perth lately and had been travelling around Western Australia (down to Augusta and up to Geraldton)… it was just great and since a friend of mine will be going soon, I had come up with a suggested itinerary for her and her husband to plan their trip.
It generally provides some details on Perth, the route south to Bunsburry, Busselton then Margaret River and Augusta. I thought it would be a good idea to share it as well since I had already done the work. Anyway its just based on my experience from my trip, so do go through your other research on the web.
Hope this helps 
Disclaimer: This is just purely based on my trip and experience, read at your own risk!