This works every time for me...
Take steel brush. Brush clean surface of live rock where frag is to be attached. The surface must be free of algae,detritus, etc. Also, the steel brush will "rough up" the surface, allowing the glue to adhere better. This is all done without removing the rock from the water. Next, remove the frag from the water, and do the same thing to the surface of the frag that is to be glued. If it is a frag plug, I usually just dremel off the peg. I also will crisscross score the surface of the plug/disc, again, so the glue will adhere better, especially for larger pieces. Rinse off any dust made by the steel brush or dremel. Next, take an appropriate amount of Aquamend epoxy (depending on size of frag) and knead it thoroughly. Pat dry the surface of the frag to be glued with a paper towel. Add superglue gel to this surface. Squish the epoxy ball onto the superglue gelled surface. Add superglue gel to the opposite side of the epoxy ball. Place frag into water. A skin will form over the superglue gel. Firmly squish the epoxy ball onto the already cleaned surface of the live rock. You may need to slightly rotate the frag as you press, so that the skin on the superglue gel is broken. This will ensure proper contact between aquamend and live rock. Hold for anywhere from 5 to 30 or more seconds (again, depending on frag size). Done!
Don't touch the frag for at least 24 hours, so that the aquamend fully cures.
HTH