Play "Third Hand High" rule to set up potential tricks for your side. Play the highest card necessary to force declarer or dummy to win with its high card.
Always play the lowest of touching cards to give partner the most information about your strength. (Play the J from QJx; the 9 from J109.)
If an honor is led and you're next to play, follow the "Cover-an-Honor with-an-Honor" rule -- if declarer or dummy leads an honor (10 through Ace), play your higher honor on it if you have one. This forces declarer to use two of his honors to capture one of yours.
1 - You have a sequence of honors (KQJx, QJ10xx, etc.) and want to be sure you force declarer to win a high card. In this case, play the lowest of touching honors.
2 - You're covering an honor with an honor, or playing your ace to win a king or queen.
3 - You're reasonably sure your ace will be trumped later if you don't take it now.
If partner made the opening lead -- and if you're in doubt about what to do when you gain the lead later in the hand -- return partner's suit. This is an especially valuable guideline to follow when defending notrump contracts.
If you want to switch suits, choose to lead through strength and up to weakness. Ideally, you want the opponent who holds the most strength in a suit to be the second hand to play to the trick. You want the hand with weakness in the suit to be fourth to play to the trick.
Don't be afraid to lead a suit that you know declarer will trump -- you can sometimes weaken his trump holding by doing so.
Don't be afraid to lead a trump. If it appears that your other possible leads will risk giving declarer a trick, a trump can sometimes serve as a safe exit.
To defend a bridge hand well, you need to be alert to -- and be able to process -- all the clues that are available. Some clues come from the bidding, which often gives you a general picture of who holds the length and strength in each suit. Your best source of information, though, is your partner, who can give you Attitude and Count Signals during the play. These signals are used by both defenders to exchange information about their length and honor holding in each suit. Here's how they work: