Get yourself a set of Spitfire 101 F4s, preferably the radial slims.
Find a real road, NOT blacktop (sticky) or smooth concrete (you'll flatspot your wheels).
Go fast.
No, go FAST.
For fuck's sake, go FASTER.
Try to slide until you slip out.
For fuck's sake, go FASTER.
Slip out.
For fuck's sake, go FASTER.
Next time, don't slip out.
The real key, especially if we're talking a full FOUR wheel powerslide, is once you can get your rear wheels sliding (easy) is to focus on your front wheels, you really do need to push, put your weight behind them. Lean back and try to be light on your feet.
When you are about to slip out, is when you pull it back in. Taking the weight off the front wheels and pushing back down on the rear and pulling it in.
As teens when wheels were too small or shot we had a hill with markers set up for slide length, it was one of those old bluestone/gravel roads that was compressed (and smooth and shiny) for years, slide like butter, if you could hang, you could get 10' without even trying; seeing sparks from bearings coming through was rad.