WordNet (r) 3.0 (2006):

buck up
    v 1: gain courage [syn: take heart, buck up]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "buck up":
   animate, assure, bolster, brace, brace up, brighten up, brisk,
   brisken, cheer, cheer up, chirk up, console, embolden, encourage,
   enliven, exhilarate, fortify, fresh up, freshen, freshen up,
   hearten, inspire, inspirit, invigorate, light up, nerve, perk up,
   pick up, quicken, reanimate, reassure, recreate, refresh,
   refreshen, regale, reinvigorate, renew, revive, revivify, set up,
   solace, stimulate, support, take courage, take heart, upraise,
   vivify