If you don't have a reservation at the state park at the mouth of the Deschutes, I'd make one now. You can usually grab a spot in the overflow area if you're there early enough during the day on a weekday, but I wouldn't count on it. I've ended up grabbing a motel room in Biggs on more than one weekend because the campground was full. Air conditioning isn't a bad thing in August on the Deschutes, though.
Be prepared for heat, sun, and wind. August days are long and the best fishing is usually early and late. Also, if you're fishing at the mouth, you'll be hiking several miles a day (at least). Pack lots of water and sunscreen. I like to backpack or mountain bike up a few miles and camp so I don't have to hike up and back each day, but both options can be brutal on a 100+ degree day. And definitely have a backup plan, the snowpack is light this year and the white glacier may make the lower river a muddy mess by mid August. Check the Deschutes Angler website for up to date info on the water and fishing conditions before you go.
On a side note, there seems to be MORE traffic on the non jet boat weekends, seems like everyone in the state with a driftboat decides to float on those weekends and boat-in campsites are at a premium.
"Perhaps fishing is, for me, only an excuse to be near rivers." - Roderick Haig-Brown