I keep accidentally pressing buttons on my iPod shuffle.  Does it have a “hold” mode?

Absolutely.

Hold down the “Play/Pause” button for a few seconds, and you’ll see the front LED flash orange.  This “locks” all buttons, putting the iPod into hold mode.  This is especially helpful when carrying an active iPod shuffle in a pocket where buttons are likely to be accidentally pressed.  Hold “Play/Pause” again to deactivate hold mode.