WordNet (r) 3.0 (2006):

tone up
    v 1: give a healthy elasticity to; "Let's tone our muscles"
         [syn: tone, tone up, strengthen]

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "tone up":
   atone, attune, brighten up, brush up, chord, fix, freshen, furbish,
   furbish up, polish, polish up, put in tune, renovate, repair,
   retouch, revive, rub up, shine, spruce, spruce up, string,
   tone down, touch up, tune, tune up, vamp, vamp up, voice