One of my favorite examples of user friendly parking is right here in Portland. The garage at the airport has excellent signage. They tell you: how many spaces are left in the garage, which direction to drive to find those spaces and then there are small lights over every parking space that are red or green (see above), alerting you from afar to an empty space. Finally, some intelligence comes to parking.
However, I do recommend staying away from the parking lot in the Lloyd Center in Portland, the complete antithesis of anything user friendly or easy to figure out. It's a parking nightmare with vague and missing signage. You can check in, but you may never leave.