1. Lead through a High Honor.
This occurs fairly often at the Bridge table. And most of the times it is preventable. First let us identify the dangerous situation. In example 1 on the right, South has taken the first 7 tricks in a 3 No Trump contract. Only Hearts and Clubs remain. South's K  is in a precarious position. As long as West (South's left hand opponent) is on lead, the K  is safe. Note: if West leads the suit or plays the A , if he has it, the K  becomes a winner. On the other hand, if East leads the suit (and West has the A ), the King is captured and the trick is lost. But how to keep East from getting the lead? 
        One way of preventing the wrong hand from getting the lead is to finesse into the safe hand. That is, if there are 2 ways to finesse, take the finesse so that if it loses, it loses to the safe hand.
        In example 1, Declarer (South) has a 2 way finesse in Clubs. He can play to the A  J in North's hand, or to the K 10 in South's hand. Which way to go depends on who is the danger hand. Again it is East. If the finesse to the A J of Clubs is taken and it losses, then East shoots back a Heart and the King is dead. And there is no heir to the throne; the Defenders take the next 4 Heart tricks for down one. So the finesse must be taken is such a way that if it loses, it will lose to West, the safe hand. So play the A  (in case the Q  is singleton) and a small Club to the South hand putting in the 10 if the Queen does not appear. If the finesse loses West will be on lead and the K  is safe; making 3 no trump.
        Another possible perilous situation is where you as Declarer have 2 suits that need finessing. In example 2 on the right. West leads the 10  against 3 No Trump. Which suit to finesse depends upon which one of the Defenders poses the greatest threat. In this example, West threatens to lead Diamonds through Dummy's King. So the Spade finesse must be avoided, because if it loses West bangs down a Diamond. And if East has the Ace, it is curtains: rods, hooks and all. Good-bye King, contract - and at a high stakes money game - Declarer's shirt.
        To remain on speaking terms with your partner, win the Club opening lead in the closed hand and take the Heart finesse. Even if it loses Declarer has 1 Spade, 4 Hearts, and 4 Clubs; making 3 No Trump.
         Another strategy to isolate and neutralize the danger hand is the hold up play. In example 3 on the right, West opens with the K  against Declarer's 4 Heart contract. East drops the J . Do you win this trick with the with the A ? The answer is YES if you want to snatch defeat from the jaws of victory. Possible scenario: you pull trump and must take the Club finesse. It loses to West who cleverly plays a low Spade. East wins with the 10  and shoots back the Q . Up with the K  which falls to West's Ace. Without even a gloat or a snicker, West plays the J  and sets the ice cold contract. You have lost 1 Spade, 1 Club and 2 Diamonds. 
        To prevent East from ever getting the lead, do not win the opening Spade lead. (Nothing is lost because you probably will have to lose a Spade anyway.) West is on lead for the second trick. Win whatever he returns (remember if a Diamond is led by West your K  is a winner), pull trump and take the Club finesse. If the finesses loses to West's K , he cannot get to his partner's hand. The best he can do is to cash the A  and let you make the contract: you lost 1 Spade, 1 Diamond and 1 Club. If he does not cash the A , you will win the return and pitch two Diamonds on the Clubs and make an overtrick.

2. Run long suit.
No Trump contracts are usually races between Declarer's long suit and Defender's long suit. The Defense has a slight edge because they make the opening lead. But Declarer has the advantage of playing 26 cards. In the 3 No Trump contract in example 4 on the right. West leads the K . The alarm should go off in Declarer's brain - if not then trade that brain in for a new one. If West has 5 Hearts he can take 4 tricks in that suit after the A  is knocked out; this plus the possible Spade finesse loss can result in down one. There are times when West's would make a short suit lead. The danger hand now would be East. How can you tell who has the long suit? Look for later discards; remember the bidding; use a little intuition; or try peeking. A mirror strategically place behind your opponent is often quite helpful. Back to example 4. 
        The K  is most likely the top of a 3 card sequence. West probably has a minimum of 4 Hearts; could be 5. Let's assume that he is the danger hand and must not be given the lead after the A  is played. This involves a two pronged attack. First, isolate West by holding up the opening Heart leads and winning the third round with the Ace. Now East has no Hearts and cannot get to her partner's hand if she is to win a trick. Second, take the Spade finesse in such a way that if it loses, it loses to East. (Remember West has the guns and bullets.) You do that by playing the A  (in case the Queen is singleton), then lead the 10  and letting it ride. If the finesse loses to East, so be it. Even the Yankees lose. But East has no Hearts (due to your brilliant hold up play) and cannot get to her partner's hand. Win whatever she returns. The J  is your ninth trick. 2 Clubs, 3 Diamonds, 1 Heart, 3 Spade; making 3 No Trump
        If West's opening lead was a low Heart like the 5, 4 or 3 instead of the King, then you might assume it is from the fourth best. West has at least 4 Hearts and is the danger hand. If the 2  is led then West has a 4 card suit and there is no  danger. Play it safe anyway. However, if West led the 7 or 8 of Hearts, this lead might be top of a short suit. Hold up and win the third round of Hearts. But now for the Spade finesse. Which opponent is the danger hand? Which way to take the finesse? Now you need the mirrors. Good luck.