Talk to the rangers in the area you want to camp in -- check out this link:
Mt. Hood Natl Forest Dispersed CampingAlso, note that there are other dispersed camping areas that are NOT indicated on that link!
You have to be self-contained -- no dumping of anything on the ground. Bring your own water, and so forth. Yes, it is a lot of extra effort. But in my opinion, the silence and the peace are priceless.
Buy a forest service map for the area. Chat with the rangers - - they are great sources of info. And Google Earth is your best friend -- you can often see boondocking sites that other folks have used in the past, so that you don't trample the bushes needlessly.