To initiate a good skid, you need to unload the rear wheel by shifting your weight to the front of the bike. The best way to do that is to straddle your stem, with your legs touching the flats of your handlebars. The farther forward you lean, the less weight there will be on your rear wheel, the farther you will skid.
Start out slow to learn the timing, and get comfortable enough to commit to throwing your weight forward.
Also, get on youtube and watch some videos. Pay close attention to the riders' motions, not just watching the skid.