From:
http://poultryone.com/articles/mareksdisease.html
"Since it is so difficult to control your birds' exposure to Mareks, (showing, bringing in new birds, airborne spread from other farms), the best course is prevention. That starts with completely sanitizing your brooders. Then consider the Mareks vaccine, which is available in a freeze-dried form through a few of the mail-order suppliers, and is easy to administer to day-old chicks. The downside is that you have to administer the vaccine within one-half hour of mixing it with the fluid it comes with (diluent), and you have to plan your hatching to accommodate the 'all-or-none' vaccination within a day or two of hatch. (After one hour of mixing the Mareks vaccine, the active virus dies and the vaccine becomes ineffective.)
You'll inject the vaccine under the skin at the back of the neck (subtecaneously). Be careful though, you could stick right through to the other side and vaccinate the floor instead of the chick!
There are certain 'B factors' contained in the blood of some chickens that make them resistant to Mareks. If you have access to a lab for 'B type' blood testing, 'B factor' birds are desirable for breeding for a 'Mareks-free' flock.
Overall, the easiest way by keep Mareks out of your flock (but not the most effective), is to promote 'age resistance' by keeping your youngsters separate from the adults and away from the poultry shows until they're over 5 months old."
From: Peter J. Brown, First State Veterinary Supply, Inc.
"If a bird that has not been vaccinated for Mareks Disease comes down with the obvious symptoms of Mareks it may be of value to vaccinate anyway. There is some research to show that some birds may recover after vaccination. This would possibly work on birds whether or not they were vaccinated before. Some research has suggested that re-vaccination 4 to 6 weeks after the initial vaccination was of value in preventing new cases of Mareks disease in high risk areas. It is important to keep in mind that birds that have been infected are shedding this virus at a very heavy rate from their follicles and it would be of value to keep feathers about your coop cleaned up. Regular spraying with oxine will go a long way in keeping this disease as well as other diseases under control."