Using a car that beats the AI by at least 3-4 seconds on a bad lap and frequently breaking your rhythym to complain on the internet about how boring the race you set up is might work too.
If that's not good enough, maybe you could try muting the GT5 audio and watching TV loudly, occasionally calling up some friends on the phone while driving.
Finding a car/tire combo that on a good lap I could gain a couple tenths on the opposition did the trick for me. But that does introduce the possibility of losing.