This is easy to combat.  You may even have the antidote in your medicine cabinet...HAIRSPRAY.  Or you can use another common product called artist's fixative.  At the art store you will find a product in a spray can that pastel artists, chalk artists, pencil/charcoal artists, etc. spray on their finished works to keep the piece from smearing called "artist's fixative."  It is clear and dries in a matter of seconds.  I use it for the exact thing you are doing.  When I'm done with a piece I put a coat of poly on the piece then I use a marker to create my thin detail lines.  Then I dust it with a coating of fixative or even cheap hairspray and put on one or two coats more of polyurethane.  The marker will not smear at all.  I guarantee it.  Here is the type of product I am talking about: http://www.jerrysartarama.com/art-suppl … ativeMatte