Race alot. Try racing alot on tracks that are semi-hard, but you can win most of the time. Once you win like 100% of the time, go on to hard tracks.
Getting better cars can help too.
Race on different kinds of maps, rally type, curvy maps, straighter type maps... just deversify yourself.
Just play the game and you'll get better. AND DON'T POST TWO THREADS ON THE SAME SUBJECT.
Try some of the license tests. They show you alot of the techiques to get around the track faster. Also play alot. The longer you play the better you get. I played about 350 hours and I'm good now with a %98 win ratio. Also some your cars suspension and transmison can be tweaked to give you a better time, sometimes cutting 10-12 seconds off a lap.
Just practice. That seems like a stupid answer but i never was and never have been bad at gran turismo, sorry.