Time trials are your friend. If you want to find out where you're losing time, or if there's a better racing line, Time Trials are your friend.
GT5 doesn't require extensive testing, but if you want to be a decent driver, you yourself will require extensive testing. I highly recommend picking 5 tracks of different styles, then run 10 laps on each in a very low powered car. A Citroen 2CV does wonders.
Yes, it will take for f'n ever to complete one lap, but it's also a much easier car to control. Once you've found all the speed you possibly can, you can work your way up to faster and faster cars, adjusting and improving your technique as you go along. Your course knowledge on those 5 tracks will also increase dramatically, which will also benefit you whenever you race those tracks again, online or off.
After a while, it will get tedious. You'll hit a wall, get sick of the tracks you chose and or the cars, and you'll want to move on. Just be sure to take with you what you've learned on your Time Trials, and you should become more consistent and faster, thus a better driver.
Finally, if you do race online, find some decent people that are years ahead of where you are, and race them. If they're good people (and assuming you yourself are a good person), you'll find some patient people that will lead you through what you're doing wrong or what you're missing.
Remember:
1. As mentioned above, fast in, fast out. Treat the apex like your significant other and get close and comfy... just before bolting (lol).
2. "What you manifest is before you." -The Art of Driving In The Rain. If you know the tendencies of your car, learn to induce and control those tendencies, rather than react to them. Be pro-active, not reactive. If you're reactive, it's already too late and the circumstance you created will lead to the consequence you feared.
3. "If it feels good, it probably is. If it feels bad... it probably is." -Kenny Brown, pro Mustang racer.
4. Visit
drivingfast.net for technique tips and advice. The articles are wonderful if you don't have the Apex book.