In addition to proper modulation of the gas, it is extremely important not to overcorrect on your steering input and/or hold the wheel in a static corrected position. 99% of people will turn the wheel too far and hold it for too long when trying to correct their direction in a skid rather than making continuous steering adjustments that match the action of the skid.

I most important tip I ever picked up in a driving course was that when the tail starts to swing out, you need to point the wheel along the path you want the car to follow (think of an imaginary cross-hair attached to the 12 o'clock position on your wheel that you want to keep pointing at where you want the car to go). Then, as the tail starts to swing back, rather than hold the wheel position (which results in overcorrection) you must smoothly and continually adjust and bring the wheel back along the path you want the car to follow.

For example, if the tail starts to slide in a corner, rather than cranking the wheel in the opposite direction and holding it until the tail starts to aggressively swing back (which will result in overcorrection), merely move the wheel just enough in the opposite direction so that you continue to steer along the path of the turn (with a slight and smooth reduction in right pedal if dictated). As the tail starts to come back, match it by moving the wheel back at the same pace and continuing to steer along the path you want the car to follow. If you do this properly and smoothly you will find the tail will come back right on line the first time with no further "tail wagging".

It's a very cool feeling when you can control a skid as smoothly as you can with this technique, but it requires patience and discipline not to do what your reflexes tell you to and yank the wheel too hard, too far, and hold it too long.