Andrew Engleson of the Washington Trail Association sent me a link to these tips:

• Don’t leave anything of value in your car. Period. Bring your wallet with you.
  Leave the laptop at home.
• Open your glove compartment and empty it. Thieves immediately
  head for the glove box. If they can see ahead of time there’s nothing
  of value there, it may discourage them.
• Consider leaving your car unlocked. Thieves are after items in
  the car, not the car itself. An unlocked car tells a thief there’s no
  need to bust a window.
• If you absolutely must bring iPods or other gear, consider
  putting them in plastic bag and stashing them in a location up the
  trail. Just be sure to remember where you put it, of course.

Of course the most obvious thing to do is catch a ride with somebody else, but that only works till your ride gets his window broken and decides it’s your turn next time.