There are several places on the site where I mention my “Bits of cardboard”, so here they are — large as life, though mine are hand-drawn rather than these beautiful creations:
I've given them different colours so that they are immediately identifiable, I've labelled them clearly, and you can see which way they're facing, not just where they are. I think the final point is vital, both for writing dances and for working out how to walk them through, so I'm baffled by people who tell me they use salt and pepper pots.
I find these useful when I'm writing dances, and also when I'm trying to understand a new dance I've picked up from a book or other written instructions. Move them slowly and carefully until you're sure you understand the pattern. I sometimes have to do this several times, so don't rush it. If you're looking at a dance with action outside the minor set you need to be aware of End-effects, so go through the sequence again from progressed positions.
I'm not saying that all callers should use these. Madeleine Smith draws a number of diagrams to show where people are at different points in the dance, which certainly makes it easier to go back to a particular instruction.
If I'm on the road without my bits of cardboard I sometimes tear up an envelope to make a temporary set — but I still keep the labelling, and the points to show which way each person is facing.