At LAX, there are about half a dozen SuperShuttle loading zones with dispatchers outfitted with handheld devices scattered around the various terminals. All you need to do is walk up to the dispatcher and tell them where you're going and they'll assign you to an arriving van. It's actually a better option than reserving, because if you reserve then you can't change your pickup time without two hours notice.
This affected us on our most recent trip, because we had reserved in advance but had the opportunity to stay at our hotel later. When we called, they said we couldn't reschedule because we "had already been assigned to a van" and that if we hadn't reserved, we could have changed our pickup time. It was really annoying and we decided never to reserve again.