Quick answer
Pass the tiles that serve none of the directions you are still keeping. How to choose on each pass, what never to pass, and how to read what arrives.
Every player agonises over the Charleston and most are asking the wrong question. It is not which tiles are bad. It is which tiles serve none of the directions you are still willing to play. Answer that and the three passes choose themselves.
The only question that matters
Sort the deal, count which card categories your tiles could serve, and name your best two directions. Everything that serves neither is passing material. That is the whole method, and it works on every pass.
This is why passing feels impossible before you have counted. Without two named directions you are judging tiles on instinct, and instinct says keep everything.
The first pass: the easy three
Lone honours that fit nothing, a single wind with no partner, an isolated terminal in a suit you have nothing else in, the odd tile in a hand that is clearly running even. On most deals three such tiles are obvious once the rack is sorted.
If you cannot find three, you are holding too many directions. Drop the weakest one now rather than at the end, because passing from a third direction is cheaper on the first pass than on the third.
What you never pass
A joker. Jokers may not be passed at any point in the Charleston, and in most groups that ban extends through the optional courtesy pass. A joker passed by accident is handed back.
Your pair prospects. Most hands need a pair, jokers cannot fill one, so a natural pair already in the rack is worth more than its count suggests. Flowers are usually worth holding early too, since several card categories use them and they are easy to trade away and impossible to get back.
The second and third passes: narrowing
By the second pass you should be down to two directions, and by the third, ideally leaning to one. Pass from the loser. The discipline that separates players is passing the tiles of a direction they have decided against rather than hedging to the end and finishing eight tiles from both.
A tile that serves both of your directions is the last thing to go. A tile that serves only the leader is safe to keep. A tile that serves only the backup goes once the leader is clearly ahead.
Read what arrives, not just what you send
The tiles coming to you are information. Three players are telling you what they could not use, and in a full Charleston that is nine tiles of evidence. A flood of one suit means nobody is building it, which makes it safer for you and also slower to complete.
If the same number keeps arriving, the table has abandoned it. That is the moment to consider whether your backup direction is actually the better one, because the tiles you need are clearly available.
The courtesy pass
After the Charleston you may exchange one to three tiles with the player across from you, and it is the only pass where fewer than three tiles can move. Both players must agree the number, and either can decline.
Use it to shed the last tile of a direction you abandoned. Do not use it to break a pair, and do not agree to three when one would do.
The four mistakes that cost the most
Keeping three directions to the end, which guarantees you finish close to none of them. Breaking a natural pair to make up three tiles, when a lone tile elsewhere would have done. Passing a tile you will want back two turns later, which is usually a flower or the second of a pair.
And the quiet one: passing on autopilot without recounting. The deal you sorted is not the rack you hold after six tiles have moved through it. Re-count before the third pass.
Put it into practice
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Passing well is just knowing your two directions. Mei, our AI coach, reads your actual deal and names the leader, the backup, and the tiles that serve neither, which is your pass.