One very useful tip, if it really worked :D

I got a useful tip from my bro. For those who have cars. Note: only for cars with keyless entry (I have no idea what they're called. They're those things attached to the manual key. They have buttons that will lock/unlock the car when pressed)

Keyless Entry

When you left your car key inside your locked car (yeah, it sucked) and you left your extra car key at home, call your home. Tell someone who pick up your call to take the extra keyless entry you left at home. Hold your phone close to the car and tell that someone to press the 'unlock' button on the extra keyless entry close to your home's phone receiver. Somehow, the unlock wave command will reach your car via the phone connection.

My bro have tried this and it worked! But some people said it was a hoax. Maybe my bro was lucky, maybe those people didn't do it the right way, maybe you just have to try it a few times to make it work. Happy trying :)