Don’t help your opponents. That’s simple enough in concept but more complex in action.

• Don’t take a finesse for your opponents. Here you have the lead early in a 4♠ contract:

Dummy: ♠ A J 10 5

You: ♠ 9 8 7

Whatever you do, don’t play spades, hoping partner has the king. Here are the other hands: 

Partner: ♠ Q 2

Declarer: ♠ K 6 4 3

Do you see what happens? If you’d let well enough alone, declarer probably would take the spade finesse through dummy, losing to the queen. By leading spades yourself, you give declarer an extra trick.

• As declarer yourself, you hold these clubs:

Dummy: ♣️ Q 8 3
You: ♣️ J 6 5

Let your opponents break clubs. If you did it yourself, you’d probably lose one club to the ace, another to the king and wind up losing three club tricks. If your opponents break clubs, they’ll have to cash the ace and king, giving you a trick with your lesser honors.

• Now how about this? You’re defending. Let’s isolate the diamond suit:

Dummy: ♦️ K J 6 4

You: ♦️ A Q 7 3

It’s important that you know what to do when diamonds are led. Any hesitation will alert declarer that you have the ace or queen or both. Unless you need a quick diamond trick, play low smoothly and hope declarer plays the king.

Sometimes your opponents help you. Here’s how East-West gave me a contract, if only I’d been on the ball. I was sitting South and dealing, with East-West vulnerable:

                                      ♠ A K 8
                                     ♥️  Q 4
                                     ♦️ J 8 2
                                     ♣️  Q 8 7 5 3
WEST                                                               EAST
♠ 10 3                                                              ♠ 9 7 6
♥️ A 9 6 2                                                       ♥️ K 8 7 5
♦️ 7 5 3                                                           ♦️ K 10 6 4
♣️ 10 6 4 2                                                      ♣️ A J

                                   ♠ Q J 5 4 2
                                  ♥️  J 10 3
                                  ♦️ A Q 9
                                  ♣️  K 9

The bidding proceeded as follows:

South              West             North            East
1♠                   Pass             2♣️                DBL
2NT                 Pass             4♠                All Pass

Opening lead: ♣️2

I won with the ♣️K (good move by East to avoid going up with the ace) and cashed the ♠A. Now I took the diamond finesse to the queen and cashed the ♠Q. At this point it occurred to me that there might be a heart trick somewhere so I gamely played them until the jack set up. In the end, though, I had to cash the ♦️A and lose a diamond.

I should have noted East’s double and what it meant. Here’s how to play the contract. On Trick Two lead a spade to the ace and take a deep diamond finesse to the 9. Why? East’s double surely showed four hearts to the king, and why not the 10?

Now the road is clear. Lead another spade to the king and take another diamond finesse to the queen. After cashing the third diamond winner, I can draw the remaining trump and break hearts. In the end  I’m home with two losers in hearts and one in clubs.