With the water-cooled turbos, no special care is needed: I do what Paul does, and drive lazily the last couple of blocks before stopping the engine. I also use synthetic (Mobil 1) so the oil does not coke up in the hot turbo unit or the oil lines. But with the earlier oil-cooled turbo, I would do the same thing but let it idle for just a few seconds (like maybe 10) before shutdown to push more oil through the unit, both slowing and cooling it. I would also definitely use synthetic, especially since the oil is the cooling medium. Letting dino oil cook inside the turbo is not the best approach.