I'm on a "B" billet right now, and about halfway through my masters. I've been in 12 years. I wish I would have done it earlier. To be honest, the added responsibilities at work and at home make getting it part-time harder as time goes on.
The best way to get a masters in the Corps is at a resident school that they send you to. There are several programs that will get you a masters. The way I would recommend is at a school such as Command and Staff, at which you basically do the Marine Corps program of study, add a thesis along the way, and BAMMO! Masters degree. However, it will be in "Military and Strategic Studies" or some such, which may not be what you want. There are some other degree programs that will lead you down some weird career rabbit-holes, as well.
I'm not going to a resident PME course anytime soon, so I'm taking a little assistance from the Corps ($250/credit-hour) and going to an extension campus on base on the weekends. When I'm done, I'll have a masters in business.
If you really want to get the degree, get it started during your first tour, AFTER you've established a good foundation on the job. This means 1 deployment, aircraft commander qual, etc.