Most of the untypical racing techniques I have learnt have been from trying to gold the licenses. When you feel that you are going as fast as you can, but are still half a second or more off the gold time, I found I usually had to think laterally and try something very different, rather than refine what I was already doing. I would never have used left foot braking in GT4 if it wasn't for the license tests, for instance. Also going through some corners in a higher gear than feels right, or going slower than I can around one corner to optimise my exit speed out of the next corner, etc. I find that I don't tend to experiment like that if the only target I am trying to beat is a personal best lap time.
I also learnt a lot about restraint and smooth driving from driving enduros in underpowered cars, because a slide or a spin is then costly.
The third thing in GT4 itself that has encouraged me to try different techniques has been from driving a lot of different types of cars and having to learn to adapt my style to the car to get speed out of it... eg. how to drive around understeer, how to drive around oversteer, etc. It all makes you rethink your braking habits, turn in points, where to place your apex, how to reapply the throttle, etc.
Some techniques specific to playing computer driving games, particularly techniques involving passing the Artificial Idiots, has been carried over from lots of time spent playing previous driving games. You start to learn to look for patterns in their behaviour and once you find one you can manipulate them far more easily. Eg. in GT4 if the AI is faster than you on the straight and is far behind you, he is going to rear end you rather than pass you if you are on the racing line. This is bad since it can often cause you to lose control. If you move out of his way and let him past, then pull in to his slipstream, you lose less time. If you are fairly close to the next corner and pull out of the way, the AI will tend to chicken out and not pass you at all, where as if you keep on the racing line he is more likely to ram you very hard just when you are about to start braking, and send you spinning into a sand trap.
Many of the techniques I practise in GT4 come from an addiction to experiences in real cars. Indoor go karting teaches you all about momentum for instance, and balancing the throttle to keep the car from scrubbing away too much speed through sliding. Minimum steering input and straightening the wheel as soon as possible after the apex of a corner etc and trying to minimise the number of corrections you have to make with the controls. Less work = more speed... That seems to work in GT4. Skills I have learnt from an addiction to driving courses have also been useful, like looking as far up the road as I can see (this makes your steering smoother, your racing line more obvious, and also works on the road.. stare at the back of the car in front of you and you will tend to follow it and react to it. Drop back a little and move aroud in the lane and stare up the road ahead of the car you are following, and that car's behaviour becomes more predictable. This is difficult when following a truck...

) and thinking ahead: concentrate on the corner coming up rather than the one you are currently negotiating. A huge wealth of techniques and ideas that I have implemented in GT4 has come from reading car magazines and books on driving techniques.
In the end though the biggest influence on my GT4 skills has simply been from playing the game very often indeed.