I always forget how to do this so here's the trick:

Mars II/(O2 xda Zinc) 
To perform a hard reset, use the stylus to reach the reset button found besides the device's camera button. 
Keep the stylus on the reset button and press the Up Navigation button at the same time.

I can't take credit for this, it was originally on this site: http://www.efsoft.it/resetdev.htm

In addition hitting the Up + the Power button when the device is off puts the device in a boot loader mode.

Tags: O2Xda Zinc Mars II Hard reset Boot loader screen