Basically, you use a light weight/high acceleration car (Dry Bomber is supposedly the best), and then starting from the middle of the track, start veering one way, and then power slide in the opposite direction. Then power slide back the other way. Back and forth. You have to be able to get the red sparks really quick after you start drifting or you'll hit the wall. Like Pale said, it's a hard move to pull off... it takes a lot of practice. I can only do it on really wide tracks like Figure-8 and Luigi Circuit, but I've seen people that seem to be able to do it constantly O_O
I enjoyed our races, there were quite a few close ones. Looking forward to racing you again sometime.