WordNet (r) 3.0 (2006):

sure-fire
    adj 1: certain to be successful; "a sure-fire way to get rich"

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "surefire":
   assured of success, calculable, certain, coming,
   crowned with success, dependable, faithful, faithworthy, fast,
   firm, fortunate, invincible, made, on the up-and-up, on top,
   out in front, predictable, prosperous, reliable, secure, solid,
   sound, stable, staunch, steadfast, steady, substantial, succeeding,
   successful, sure, triumphant, trustworthy, trusty, unfailing,
   well-founded, well-grounded