Using a theraband, you can perform a proper warm up for the wrist and forearms in just a couple of minutes.  These types of bands can be purchased in rolls with different levels of resistance.  Anchor the band or have a partner hold the other and and perform the following moves: wrist flexion, wrist extension, pronation, supination, ulnar deviation and radial deviation.  Perform each movement for 15 reps on both sides.

Check out the video for a demo