I've studied Shorin-ryu karate from Okinawa, Tai kwon do, and Tai-ji. I agree with TxDoc about books/DVDs for martial arts, yoga, and Pilates. The practical side of having a teacher is that you will (hopefully) be taught the correct postures and moves, drastically reducing the chance for injury. The more spiritual part about having a teacher is harder to articulate, but just as important. For all of the martial arts, the philosophy is very important, not just the physical part. You can only learn so much from a book.

I stopped doing martial arts in 2001 when I moved and was so busy with my job. I did not take the time to find a dojo, although I imagine there were plenty around (it was a big city). I did pick up Tai-ji after a few years with a private teacher who also happened to be a high-school friend. I traded landscape design services for tai-ji, but again, I got so busy and then I moved again.

I might take up tai-ji again, as it was the martial art that spoke to me with the most memorable voice.