Only You
I'll start with the simplest and most unambiguous of situations. That is when only you can guard a suit or perhaps only one card that declarer could otherwise turn into a winner. You must retain your guard over that threat (as long as you're certain declarer has an entry to it, and insofar as is practical when declarer might have an entry to it). If in so doing you uncover a winner for declarer in another suit, then you were legitimately squeezed, and there's no shame in that. The oft-seen error is that a defender uncovers his protection of that suit with the upshot that both defenders are protecting another suit and declarer waltzes home in a suit neither defender is protecting.
I will start here with a totally unambiguous situation and in later examples cite situations from actual play:
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 A Q 8 2 |
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| J 10 6 4 |
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West goes low on declarer's lead to the queen. Declarer leads back to the K, East showing out. Now declarer knows and you know that the suit was originally 4-4-4-1 and any discard by you would give declarer a fourth winner in the suit. You must guard against that, whatever else you do. Discard an ace or otherwise high card in a suit? Absolutely, if your choice comes down to that.
Your partner may or may not then have the king. But you know he can't protect the club suit. If declarer then shows up with the king where you've thrown the ace, then you were legitimately squeezed and, as mentioned above, that's not something you need say "sorry" to your partner about. But sometimes you'll find your partner does have the king.
In that case, you haven't broken up a squeeze. Rather, you have refused to fall into a trap where there was no squeeze.
Here is a case with a touch of ambiguity:
 A K 10 7 5 |
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J 9 6 3 |
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Declarer cashes the A K, dropping your partner's doubleton queen, and ruffs a round. Now you know you're the only one who can protect the fourth round of the suit, and as long as you're certain declarer has an entry to it, or as long as your other cards are completely inconsequential, you'd better hang onto your jack. The crunch comes when you have to part with something valuable and you're not certain whether declarer has an entry to dummy or not: say, an as yet untouched side suit shows Q x x.
Obviously, the more tenuous the situation, the less one can advise ahead of time in writing. One can only say remember the bidding and weigh what declarer is likely to have and do your best. Above are links to illustrations of this principle in play.