I am finding that numbers are my friends.
The only way to prevent mistakes is to count stitches, and re-count after every row of work.
Now, generally, when I get to working, my mind sometimes drifts. Counting more than, say, 20, I am never 100% confident that my count is Really correct. I experimented with a couple of things to help me keep track of threads. I thought about placing ‘marker beads’. This would mean sliding on the required number of beads ahead of time, sliding them along as I measured out, and leaving a bead in place at the appropriate place. The problem I anticipated was that any knot or slub on the thread would mean that the rest of the beads could not pass. I opted for placing little ‘marker threads’ every 100 threads. We re-counted at each 100, to make sure this marker was accurately placed, and to be keeping accurate account of the number of warp threads.
As soon as I began to ‘sprang’, however those markers proved impractical. They got tangled, and created tangles.
So, I’m relying on counting and looking, checking each row twice.
Breaking down the pattern into smaller pieces helps. Indeed, most of the patterns on this replica sash do that.
And let me stress again, keeping the threads well separated goes a long way toward error-prevention, as well as making it possible to push each work around to the other side.