Generally its not a good idea to lead a Wizard as it allows other players to set up their hands by discarding unwanted cards.

If more tricks have been called for than there are tricks available the Wizard should be used to win rounds in which the trump suit is led. This removes winning tricks from the opponents hands and increases your chances of winning additional tricks.
When fighting for tricks don't play a Wizard simply to win a trick. It should also remove cards from opponents' hands with which they had hoped to win tricks.

Both W & J can be used to good advantage in situations in which you have already broken. Good players are prepared to take an additional loss of 10 or 20 points if in so doing they can prevent opponents from making their contract and scoring high points.

Most experienced players agree that Jesters are more useful than Wizards. Jesters are rarely a problem whereas sometimes you may find it difficult or impossible to unload an unwanted Wizard. Jesters hold both offensive and defensive potential because they allow you to protect a card that you dont want to play. For example you may not wish to play a singleton King on an Ace lead or you may not want to discard a Trump card that you need. The Jester allows you greater flexibility in playing your hand. Having said this does not diminish the great value of the powerful Wizard card.