Knock Out Their Entries First
When you must lose the lead twice in no-trump, first knock out the entry held by the defender with the long suit.
In no-trump the defenders try to set up a long suit and then reach it. When you have to give up the lead more than once, the order matters. Knock out the entry of the defender with the long suit first, while you still have a stopper in their suit.
Why it works
A long suit is only dangerous if its owner can get back in to cash it. If you remove that player's outside entry while you still stop their suit, they can set the suit up but can never reach it.
A worked example
West overcalled 1♥ and you are in 3NT. West leads the ♥Q.
Hearts: West ♥ Q J 10 9 8, East ♥ 7 6, Declarer ♥ K 4 2, Dummy ♥ A 5 3
You need to knock out two aces, the ♦A and the ♣A, to make nine tricks. From the overcall you place West with the ♣A and East with the ♦A.
Win the first heart. You have one heart stopper left.
Wrong order: play diamonds first. East wins the ♦A and returns a heart, knocking out your last stopper. West's hearts are now set up, and when you play clubs West wins the ♣A and cashes three hearts. The defence takes two aces and three hearts: one down.
Right order: play clubs first. West wins the ♣A and clears hearts, but that is the end of West's entries. When you play diamonds, East wins the ♦A and has no hearts left to return. West's hearts are stranded and you make your contract.
Four common mistakes
Starting with whichever suit looks strongest instead of thinking about entries.
Ignoring the auction, which often shows who holds the missing high cards.
Taking your sure winners first and leaving the defence more time.
Forgetting to count the long-suit holder's cards in their suit.
When the rule does not apply
If the long-suit holder has two outside entries, you may not be able to cut them off, and a hold-up or a different plan is needed. If you can't tell which defender holds which entry, use the bidding and the opening lead as clues. Sometimes you need a different approach, such as keeping the danger hand off lead altogether.
Key takeaways
Remove the long-suit holder's entry first.
Do it while you still have a stopper in their suit.
Use the bidding to place the missing high cards.
Plan the order of every suit you must give up.