Well, there was a car parked at the exit of a parking space in the building narrowing the space quite a lot for other cars to exit and, luckily, it had enough dust on it's windshield so I wrote the message with my finger on it's windshield and to my surprise, i haven't seen that car parked at that spot again : ). ..
And as for myself, i keep a printed note in my car saying "I am sorry if i am blocking your way, please call me (my number)" so that if i cause inconvenience to others it can be solved promptly : )