Originally Posted by Just Jan

Depends on the ruler. Some specialty rulers are worth saving due to the price to start with! If possible use a fine tip perma pen and the non shiny scotch tape. If you can write on the ruler, go over the marks, then put the tape over your marks to seal it in. If you can't write on the ruler due to it being glossy, put a strip of the flat finish scotch tape on the ruler, write on the tape then seal it with another piece of tape. Give the ink time to dry before adding the tape over it so it won't smear.