WordNet (r) 3.0 (2006):

get cracking
    v 1: start to be active; "Get cracking, please!" [syn: get
         cracking, bestir oneself, get going, get moving,
         get weaving, get started, get rolling]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "get cracking":
   attack, dive into, enter on, enter upon, fall to, get busy,
   get going, get with it, go to it, hop to it, hotfoot, hump,
   hump it, jump to it, launch into, light into, pitch in, pitch into,
   plunge into, sail into, set about, set at, set to, set to work,
   shag ass, shake it up, snap to it, start in, step on it, tackle,
   take on, tear ass, turn to, undertake, wade into