You can use a two-part reef-safe epoxy or super glue. The epoxy is sold in many LFS
(PetSmart carries one with the Instant Ocean label in my area for $6.99). When you buy it, the outside is one color with the inside another. You pinch off a piece and knead it until it's all one color and the consistency of Silly Putty. Then just put the frag or plug where you want it. When the epoxy hardens, it's close to concrete.
Super glue (cyanoacrylate) might scare you, but it's actually very reef safe. The gel type tends to work faster. Just dab a little bit on the bottom of the frag and hold in posistion until it begins to set.