I could answer in one word.
Practice.
You can watch all the videos and read all the notes, but at the end of the day if you cant ACTUALLY do it then you've just wasted time. For example, you can watch all the videos of some young gun doing all these fancy moves with a soccer ball. But you go outside and try the same thing and it just will not happen.
For GT5 what I can advise doing is getting a good all round car. I highly recommend a road car that you know you can drive well. Personally, I'd use my Evo 9 RM.
Turn on the driving line in the menu and for the first 2-3 laps try and stick to the line at a good pace. After that, drive a little harder so your really testing the breaking points and trying to kiss the apex's. Once you've done a few laps like that, exit, turn the driver line off and do the same again. 2-3 laps at a moderate pace and then go faster and try and meet the apex's.
I've tried and tested this, maybe it'll only work for me but its an opinion you want right? I used to DESTEST Laguna Seca. Really really hated it. But after doing so many laps in GT3/4 and practicing on it to get decent laps times I soon found myself in a proper simulator enviroment racing in a tight battle with an ex rally driver. That night is when I can say Laguna became my favourite track ever. I may have lost but the fact that I knew the track kerb by kerb, the passing points, and how to take the Corkscrew I knew the hard work had paid off. (FYI, I did this race in a place here in NI called Room2Race, 16 proper racing cockpits all hooked up with rFactor software to run simulated races.)
The next thing is the car setup. I wouldn't say Im a good tuner, I know what it does but it often confuses me at the same time. I do however have a "standard" setup that I can put on all cars and they will handle better for me I find.
The final tip I can put in, try and do these practices in a lobby online or just your own lounge. The fuel and tire wear are great to learn, and you'll soon find that the optimised racing line / better car control is actually going to be a much better way of saving fuel and your rubber than driving slower.
