Don't Lead Trump for Your Opponent -
When your opponent calls the suit for trump, and it's your turn to lead, it's best not to lead trump. You might want to lead an off-suit ace instead. This way, you're not pulling trump from your partner's hand. The only exception to this rule is if you think you have enough trump in your hand to euchre the other team.
Source of text: http://voices.yahoo.com/tips-play-euchre-like-pro-16270.html?cat=7