Allright, first of all you can't make mistakes. With a highpowered car you can handle understeer and such with the power, but with an under 300bhp car messing mid-drift usually results in the loss of speed or a spin.
Power over drift is out of the question in most high speed corners in GT3, so you need to use other techniques. Clutching would work perfectly, but there's no clutch in GT3 so you have to use the gears instead. When you enter a corner, brake hard and if it's a 2nd gear corner, leave the gear at 3rd and hit the accel, then downshift to 2nd to get a quick "boost" and whoo, you're sliding.
Just don't get too much sideways or you'll lose speed.
I'm too tired, so i'll let the others give you more pointers.