It depend the car and track, i first try to understand what the car like, and the first thing is to run laps just enought to know the circuit, then i stop, the next day or session later it become easier and i know where to start to push, i just look for the shortest line, the closest you can run to the apex the faster you will go (normaly) sometimes you need to run wide for the next few corners.... and try to find where you need to brake to have the faster exit, sometimes braking late will ruin the exit and you lose more than you win.
Im a little busy, have to go to work, but i can try to explain better in PM when ill get back from work.
The most important thing in GT5 you need to brake once, release throttle, trailbrake if needed and accelerate as soon as possible all that once, if you have to wait between you lose time, coasting dont happen a lot except if you have no choice. Then try to find the line that will allow a fast exit.
Goodluck!