WordNet (r) 3.0 (2006):

beef up
    v 1: make strong or stronger; "This exercise will strengthen
         your upper body"; "strengthen the relations between the two
         countries" [syn: strengthen, beef up, fortify] [ant:
         weaken]

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "beef up":
   accelerate, aggrandize, aggravate, augment, blow up, boost, brace,
   brace up, buttress, case harden, complicate, concentrate, condense,
   confirm, consolidate, deepen, double, enhance, enlarge, exacerbate,
   exaggerate, expand, extend, fortify, gird, harden, heat up,
   heighten, hop up, hot up, intensify, invigorate, jazz up, key up,
   magnify, make complex, multiply, nerve, prop, ramify, redouble,
   refresh, reinforce, reinvigorate, restrengthen, sharpen, shore up,
   soup up, steel, step up, stiffen, strengthen, support, sustain,
   temper, toughen, triple, undergird, whet