Where do you live?
I used to live in Nannup, know of some of the trails there, there was some great short DH type trails close to town. I know of a local artist that works on some of the trails and would know a lot of them (he's probably still there).
If you're into DH then you probably know of the trail which is along the Nannup-Balingup Rd as it is used for racing sometimes.
I never got to exploring further afield but sounds like there are a lot of good ones around Pemberton / Margs. Pile Rd / Collie as well.
The LBS in Margs might have some good info, PMBC people would probably be familiar with the ones further up (Welli Mills, Pile Rd etc).
I'm sure there's quite a few people on Rotorburn from that region of WA who can give you much better advice than me.