WordNet (r) 3.0 (2006):

think twice
    v 1: consider and reconsider carefully; "Think twice before you
         have a child"

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "think twice":
   back down, balance, be careful, be cautious, debate, deliberate,
   demur, dither, falter, fear, go on tiptoe, hang back,
   hang in doubt, hem and haw, hesitate, hover, hum and haw, jib,
   make haste slowly, pause, ponder, pull back, pussyfoot, retreat,
   scruple, shilly-shally, shy, stick at, stickle, stop to consider,
   straddle the fence, strain at, take it easy, think twice about,
   tiptoe, vacillate, walk on eggshells, withdraw, yield