The simplest solution I have found for this is to print a hex grid onto a transparent sheet of acetate. You can then overlay the acetate over any printed map.
Here is a link to a hex pdf generator
http://incompetech.com/graphpaper/hexagonal/
This is not as simple but allows for more flexibility of hex interior (dot, cross, notation)
http://www.nomic.net/~uckelman/mkhexgrid/