The easiest way it to use smart phone with GPS locator. When you park, simply record your location and when you need to find it use navigator which guides you to its place. If you don’t have such phone and want to build one, follow msuzuki777 instructable where he builds car tracker.
Device is pretty simple when building using modules. So here he uses Ublox NEO-6M GPS module, LSM303DLHC magnetometer, custom Arduino, Nokia 5110 LCD and Lithium battery. This locator simply remembers coordinates where your car is parked. When you need to find simply turn on and you will see how far you car is and what direction you need to go thanks to magnetic sensor based compass. You can find more uses of this devise. For instance when hiking – you can remember start location and let it guide you home when done.