There are occasions when as Defenders you can be really aggressive, setting up your own winners before the Declarer is able to do that. This situation arises when you hold a suit which contains a sequence or even two touching Honours.
However in many cases you don't have such suit holdings. Then it is best to wait for Dummy to go down on the table and figure out what Declarer's strategy is going to be.
Here are five common strategies to follow.

1. You hold a sequence or touching Honours in a suit, or partner made an Overcall
   Action : Be aggressive, lead that suit and try to establish your winners, before Declarer does.
   If this option is not available, wait for Dummy to go on the table and determine Declarer's strategy.
   
   
2. Does the Dummy have shortage and is Declarer going to ruff in Dummy ?
   Action : Lead trumps if possible at every occasion you gain the lead, reducing or depleting Dummy's trumps.
   
   
3. Is there a long suit in Dummy which Declarer may try to develop ?
   Action : Try to knock out Dummy's entries in the other suits, so that it becomes isolated from Declarer. If Declarer leads Dummy's long suit, hold up your stopper (if you have one) until you believe Declarer has run out of cards in the suit.
   If it is unlikely that you can prevent Declarer from accessing Dummy it is best to cash your own winners before Declarer discards his losers in your suit(s) on Dummy's established long suit.
   
   
4. Are the Honour cards equally divided over the four hands ?
   When there are isolated Honours in Dummy and in your own hand
   Each time one side is the first to lead a new suit in this situation the other side will benefit with an extra trick they would not have made otherwise.
   Action : Avoid leading a new suit. Perhaps you have already given a trick away with the opening lead. So the harm is done and you have probably not much to lose anymore in the suit : keep leading it.
   Alternatively if Declarer leads a new suit and you win, lead it right back to him. Every time you force him to lead a suit first you may gain an extra trick.
   
   
5. Declarer has a void or singleton in your long suit
   Action : Keep leading your suit, it forces Declarer to ruff and in doing so reduces his trump length (The ruffs will not gain him extra tricks (see Lesson 12).
   If Declarer has to ruff two or three times he may well end up having less trumps then you or your Partner. Result : he loses control of the game.
   This strategy is especially effective when you hold 4 trumps your self, or when you hold only one, in which your Partner is likely to hold 4 trumps.