To correct for any skid, let up on the gas and gently turn your wheels in the direction you want to go.
Be careful not to over correct and do not re-apply the gas until you're headed in the direction you actually want to go. Panicking and steering sharply into the turn will only reduce control.
If you're fishtailing or sliding, it means you're already going too fast. Reduce your speed so you won't need to worry about this! Most high-speed slides are difficult to correct successfully, but if you're caught off-guard and begin sliding, turn your wheels in the direction that the rear of your car is sliding. It helps to look with your eyes where you want the car to go, and turn the steering wheel in that direction. It can be easy to steer too far, causing the car to slide in the other direction. If this happens (called overcorrecting), you'll need to turn in the opposite direction. Read more about correcting a slide here. http://icyroadsafety.com/tips.shtml