I have to check for apps!
Using a Running Yarn Marker
I use yarn similar to the way she does it in this video but I wrap it around a stitch. With my current project I need to keep count of rows in a 2 row repeat and I'm wrapping a stitch every 20th row. In other projects I use a different number or a particular row repeat, maybe the first, maybe another. It would be possible to use 2 strands of different colors to keep track of different rows.
Maybe I should just go get some M&Ms.