With the French Open about to begin, it will help players to watch how the pros use their slide to cover the court and keep their balance.
The best place to start when trying to improve your sliding technique is to do some practice movements with your racquet in hand but without hitting the ball. Try starting about two feet behind and in the center of the baseline-run towards your forehand side and turn your shoulders as if you are getting ready to swing.
After two or three steps, push off with the inside foot (left if you are running to the right, right if you are running to the left) and try to keep your weight evenly on your outside foot (right if you are running to the right, left if you are running to the left). Practice starting your slide three or four feet from where the ball and the racquet would meet and see how far you go.
You should try to stay away from the lines when sliding as this will really stop you suddenly or cause you to lose your balance. You can also practice sliding while running towards the net with the same idea.
It is extremely difficult to teach someone how to slide on clay if they have not done it much but if you play on clay a lot and are willing to accept the sliding idea, you will get better the more you challenge yourself to do it.