WordNet (r) 3.0 (2006):

buckle down
    v 1: work very hard, like a slave [syn: slave, break one's
         back, buckle down, knuckle down]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "buckle down":
   be determined, be in earnest, be resolved, buckle to,
   cross the Rubicon, die fighting, die hard, endeavor, go all out,
   go for broke, hump, hump it, knuckle down, lay to, mean business,
   never say die, ply the oar, spare no effort, take the plunge