With the weather heating up, you're probably spending more time in the great outdoors than in the car. Or rather, you may be driving to the best parts of the great outdoors and leaving your car unattended for hours at a time. And because warm weather also brings a rise in car-related crime, you may come back to your car, happy and sunburned, and find that someone has broken into your vehicle.
There's no sure-fire way to avoid having your car broken into and most car crime avoidance "tips" are just common sense. Don't leave valuables out in the open. Don't leave the doors unlocked. Choose to drive a vehicle so decrepit that thieves will simply mock it and move on (this is my father's strategy).
Even if you don't leave valuables in plain sight, though, your car may still be a gold mine for thieves. According to this article in the Globe and Mail, sometimes thieves don't care about taking your iPod - they really just want your vehicle information.