To my knowledge you cant breed for resistance to Mareks. The only thing you can do to affect your experience with Mareks is vaccinate. If you have seen Mareks and the next year you didn't have it you are just lucky. This just my opinion through experience.
Matt
It is well known that some chickens have natural resistance to Mareks, so why wouldn't one be able to breed for resistance? It seems to me, if you have a group of birds and some have come down with Mareks but not all, if you breed the ones the didn't get it, you should produce more birds that are resistant. It is logical to assume they would all have been exposed given that it spreads readily through dander.