WordNet (r) 3.0 (2006):

nail down
    v 1: succeed in obtaining a position; "He nailed down a spot at
         Harvard" [syn: nail down, nail, peg]
    2: define clearly; "I cannot narrow down the rules for this
       game" [syn: pin down, peg down, nail down, narrow
       down, narrow, specify]
    3: make final; put the last touches on; put into final form;
       "let's finalize the proposal" [syn: finalize, finalise,
       settle, nail down]

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "nail down":
   afford proof of, ascertain, assure, balance, ballast,
   bring home to, certify, cinch, clear up, clinch, counterbalance,
   decide, demonstrate, determine, dismiss all doubt, ensure,
   establish, find out, firm, firm up, fix, follow, follow from,
   freeze, get at, have a case, hold, hold good, hold water,
   immobilize, insure, keep, make certain, make good, make no doubt,
   make no mistake, make out, make sure, make sure of, pin down,
   prove, prove to be, prove true, reassure, remove all doubt, retain,
   see that, see to it, set at rest, settle, settle the matter, show,
   sort out, stabilitate, stabilize, steady, stick, transfix