Alternatively, once you match somewhere you can look up that person's ASTRO poster/oral presentation and act like you were there at the time.I was an MS4 attending ASTRO in Atlanta last year and I enjoyed a few sessions. Most of it will be over your head. Trying to network is pretty much pointless -- the people you'll want to be in touch with are too busy to really connect with in most cases. There was a resident/student mixer which was a nice idea but kind of awkward in practice.
On the flip side, if you do interview at a program where one of their faculty presented something major at ASTRO, it's nice to be able to chat about the talk on your interview day.