Some time ago I have come across a success story of "Car locator" app for Android. I never bought it although sometimes I need it desperately as I am the kind of person who often times, if not always, forgets where he leaves his car. My personal rule of thumb about paid apps is that I buy absolutely necessary apps. While this one seems like necessary one, I never got it for some reason.
A week or so ago, on my way to Florida, I managed to score a Daytona 500 ticket as I was passing by Daytona area, so I went to watch the race. As I entered a parking lot I realised that no matter how hard I try to memorize, I will never find my car without some sort of assistance. That's how bad parking lot was organized. I believe it was parking lot 7 in Daytona Beach,FL. All the while, I couldn't afford to get lost, as I had to meet my girlfriend in West Palm Beach airport who was flying down from Boston later that evening.
With time of an essence, I had to rush to the raceway, while doing some improvisations to find the car easily after the race:
- I parked the car in my desired spot;
- made sure my current location on Android Google Map was current;
- clicked and held on my current location until it loaded a custom marker;
- then, I clicked on custom marker and chose "Share this place";
- next, I emailed myself my own location.
Then, later on when I needed the car, I opened my email and clicked on the google map url that I emailed myself. Car was exactly on the spot that map brought me to. I was on my way shortly and on time. 
This can probably be done on iPhone as well; I did this on Android phone.