Quick answer
Pass the tiles that serve none of the directions you are still keeping. Sort the deal, name your best two card categories, and send what fits neither, starting with lone honours, a single wind with no partner, and isolated tiles in a suit you barely hold. Never pass a joker, because jokers may not be passed at any point in the Charleston or the courtesy pass. Keep natural pairs your direction uses, since a joker can never fill a pair.
The useful Charleston question 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. Sort the rack and look for these first.
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 the ban runs through the optional courtesy pass as well. A joker passed by accident is handed back.
Your pair prospects. A joker cannot fill a pair, so a natural pair in the rack that your direction uses is worth more than its count suggests. Weigh flowers the same way, by whether your direction uses them.
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 is passing the tiles of the direction you have decided against, rather than hedging to the end and splitting your tiles between two hands.
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. Each of you names a number and the smaller one is used, so either of you 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.
Four passing mistakes
Keeping three directions to the end, which spreads your keeps across three hands when only one of them can win. 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, such as a flower your direction uses or the second of a pair.
The fourth is 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
New to the game? Start the course at Lesson 1, Meet the Tiles. The first three lessons are free and need no account.
Already play? Have the coach read a real dealt hand for you, free. It names the direction the hand goes for, what to keep, what to pass, and the reasoning behind each call.
Passing well is just knowing your two directions. Mei, our AI coach, reads a dealt hand and names the leader, the backup, and the tiles that serve neither, which is the pass.