Oops, it seems that I made a mistake. The thread that I saw was for the Nissan 350z.
Anyways here's how I drift the Mustang GT:
First off: settings. Set the weight balance to the rear. This is the most important because otherwise it will understeer.
Second: The technique that I find works best is a mixture of power-over and feint drift. Here is an example of how I approach a RIGHT hand turn in the Mustang GT '05:
1) Power-over like for a left turn -> countersteer.
2) Add countersteer, get it to whip around so that it's now facing the right way for the turn.
3) Countersteer.
4) Add a lot of throttle and enough countersteer to keep it sideways.
Practice whatever works for you with the mustang, but don't get discouraged, it's quite difficult to get sideways.