My track is also "floating" in the ballast. I started with a fill of topsoil to within a few inches of my final height, tamped down and compacted into place. I then finished it with crusher fines, again tamped down and leveled. The track was then layed in place on top. More crusher fines were then poured in around the ties and lightly tamped down and the excess brushed off. I find the fines lock in place real nice, especially after they've been wetted down and allowed to try. It's not so that you can't pull it out, but well enough that I'm not afraid of it moving around. As long as the ballast is supported well on the edges, I've not had any trouble with it washing away.
Edit: You can also try searching this forum for "ballast" or "track support" -- I'm sure you'll come up with (quite a) number of earlier threads!