The main concern with stretchy elastic, such as Stretch-Magic in bracelets  is the elastic snapping or the knot coming undone.  The best way to ensure the latter does not happen is to apply a strong, yet invisible glue directly on the knot.  If the bracelet unravels, string the beads back on and tie the knot again, then apply the glue.  I use G-S Hypo Cement which has a very fine pinpoint applicator.  Try not to abuse the bracelet, as a good yank could snap the cord, just as with any jewelry.  I offer to repair any broken bracelets, and will only charge $ .10 for any beads that need to be replaced.  You can see my return policy for further information.