Saunter - the Speed 12 has tons of power and will spin the back wheels on demand (assuming LSD and suspension are correct). GT5 scores drifting on "turning" the car with the throttle - in other words, its not about sliding sideways as much as it is turning the back end of the car with the accelerator and maintaining angle. Think of it as less focus on e-brake drifting and more focus on burnout drifting.
Also, speed may be your enemy - its not about how fast you go but rather angle maintained throughout the drift. Watch the high score replays, for the most part they aren't traveling as fast as you would think.
Assuming that you've got a car with decent FR power, try setting all the LSD settings to 60 at the rear and set your camber high. I find that higher camber in the rear than the front works best for me - the wheels may look a little funny but the rear end will kick out a lot easier. Also, use a manual transminssion and try entering turns in a higher gear but lower speed.
If you cant find a Speed 12, try a Viper. Finally, try using comfort tires - I use Comfort Medium on the front and Comfort Hard on the rear.
At least this is my opinion. There are much better drifters here, I'm an average driver at best but I can get 19,000+ on Tsukuba and can score in the top 10% of every drift trial.
Just play around with the cars and entry speeds until you get a feel for things. Try just turning into the corners (don't bother with countersteer yet) and let the car start to move sideways on its own; then start working the throttle and wheel until you start to develop a feel on how it works.
ETA - also, turn all aids off and set ABS to 1. If you haven't tried it yet, you may want to turn controller sensitivity to 7 as well. It won't be as smooth but it will let you get better countersteering from just bumping or tapping the stick.
