Now this is a cheat that I’m sure most weavers do! Still, worth mentioning I think.
When I work a repeating pattern where I might lose track of where I’m up to, I put a marker at the selvedge edge when I start a new repeat. This is usually just a little bit of contrasting thread, but I’ve been tempted to deploy some moveable knitting markers (glorified safety-pins really). By marking the start of a repeat I can always count to/from that marker if I get lost.
I suppose you don’t have to use a moveable marker – you could leave them in if the selvedge is going to be hemmed/sew etc – but I like taking a break at the end of a pattern repeat to move the marker. It gives my concentration a rest and also prompts me to consider a cup of coffee, or whether it’s midnight and I should pack it in and get some sleep!