How to draw a grid over and image that won't be printed?
A low tech solution is to print a grid onto acetate, cut it out and just lay it over the image you want to copy. This has the advantage that it will work on any medium and is quick and easy to use once you have made your grid. You could also have different grid spacing for different applications. 

You could also glue it into a card mount to make sure it lays flat.