I think the problem with these kinds of threads is that everyone drives diffentaly, has a diffrent approach so help and advice can be contradictory or not very useful, but I'll put in my own experience in an effort to give some clue to those still struggling.
My best time so far is a 1:07.385 I'm only .250 off (or there abouts gold), I've been working on it for about a day or so and here is how I've managed to improve from my original time of a 1:10.xx. For the record I'm using a DS3.
First off learning the track, it's the biggest and hardest part, I use the racing line as I feel that it's the quickest way to learn, I use the suggested breaking areas and other on track things as refrence points and the just see how far I can push it.
With this car I found that I lost the most time in the slow right hander of T2 so I put TC to 1.
Then I just practiced, you should see an over all improvement on your times the more times you do it, sure it's not going to be faster everytime but over the course of an hour or so I went from 5-6 seconds off gold to 7tenths.
The biggest key is that once you stop improving, take a break and do something else!
I had a quick blast this morning before work and managed to shave over half a second of my previous best, I'm almost to gold... Which should highlight that it's not that difficult, it just requires work and practice