I work on copies of original charts and make markings on them to organize myself better.
For big, white and black charts I use some highlighter pens - different colors to highlight repeatable stitches and orientate better, especially when the symbols are very similar.
When I am finished with a part of the stitching I mark/cross it with one color.
I use the thread parking method and to be sure where the thread is parked I put a little dot on the chart, in a corner of a cross stitch cell where it comes out from the cloth.
So after the work is finished the charts I worked on are for no use - they are very messy - that is why I keep original charts clean.