L2 - Brake
R2 - Accelerate
Left analog - Steering
Handbrake - O
Steering Senstivity - 7
If you use Manual Transmission I suggest L1 to downshift and R1 to upshift.
EDIT: What are your controls?
Not to deter you from what you prefer, but L1 and R1 aren't very good choices for the shift buttons.
When you do this, you can't keep on the gas and brake when shifting. The gas isn't very important being that the game cuts the gas when you upshift (as it should, replicating clutch disengagement) but because it does it faster than you can release and re-engage the gas, it still makes a difference.
What is important is when you're braking. When you downshift with L1 while braking, you have to take your finger off the brakes to hit the button. For this little bit, the car is not braking, but only slightly decelerating. This change in the rate of decleration, and the subsequent change when you get back on the brakes, can disrupt the balance of a car when trail braking.
It's more pronounced in real life, which is why trail braking is discouraged among novices, but it is still present in GT5.
As for my setup:
Left Joy - Steer
Right Joy - Look left/right and reverse
Triangle - Upshift
Square - Downshift
Circle - Headlights or wipers?
X - E-brake
L1 - Look Behind
R1 - Flash/Toggle Lights
Select - Change view
L2 - Brake
R2 - Gas