My anubias has trouble staying in the sand until the roots actual start winding their way into the substrate, but still, they're not rooted... just growing onto lots of tiny, tiny rocks. Haha.
What I find works best is to secure them to small rocks (ones I find outside and rinse off well work fine, since they're so small) with some thread and then to bury the rock and roots. This way, the roots grow into the rock, and eventually into the substrate itself. Securing to driftwood looks extra nice, too.