There's an app for that. Both the Android Market and iTunes store offer apps that will assist you in find your wheels quickly. Park you car. Hit a button on your GPS enabled smart phone. Your cars location is remember. When you're ready to leave, open up the app and it will direct you back to you car. I haven't tried many of the apps. MyCar Locator Free for Android is good enough for me. MyCar Locator also tracks how long you've been parked. That will help you avoid getting parking tickets if your car is resting in metered parking.
There is another solution, which is not as high-tech. When you find your parking space, take a picture of the area surrounding where you parked. Be sure to capture an easy to distinguish landmark or street signs in the frame. When you're ready to return to you car, pull out the camera and look at the picture you took earlier. The GPS apps may not work in a parking garage, but a camera sure will.