You can get really small plastic hair ties. Tie one of them underneath the bead and it should stay in (they're easy to get out too)
yeah i have dreads too, i havn't got any beads in them yet though, i'm gonna make some beads outta phimo
yea little bands you tie one first under the bead and loop it and tie it on top of the bead so it sits either side of the bead. Or the can sew them in when you sew up the bottom of braids (if you dont have any bands just use needles and threads see, works just as well if not better)