My SOP is to pressurize the keg to 20 psig (30 psia) and push the sanitizer out to get the dip tube, beer line, and faucet sanitized, then blow down using the PRV and repeat the pressurize/blowdown process twice more. That *should* reduce the O2 from 21% to <1%. Attach the auto-siphon to the liquid post, open the PRV to start the flow, finish racking the beer, and then pressurize/blowdown three more times to purge the headspace. Once you actually play around with a DO meter, it's shocking how much purging is necessary.
CTSP (chlorinated trisodium phosphate)
5 Star's CMC, in case anyone was wondering. Awesome stuff.