2. Play the waiting game

You lead J♠ against 3♦ doubled. Your partner reads this for a doubleton (thinking declarer is unlikely to have four spades), and plays Q♠. Declarer wins K♠ and plays K♦. You win A♦ and continue with 9♠ to partner's A 10♠. Partner then plays a fourth round of spades which declarer trumps with 9♦. If you overtrump with J♦, you will get 3♦ one down - not as good as making 3♥ yourselves. Instead of trumping with J♦ which will always win, discard. This way, you will not only make your J♦ later, but also your promoted 8♦ for two down and a great score.