At first, during the first year I believe, I called my Sensei by his given name. The strange thing is over the years I have quite a respect for him that I and my fellow students call him Sensei on and off the matts.
This is not true however for other yudanshas. Not that I disrespect them. I call them by their given name if their about my age group and call them sirs if they have quite an age difference with myself.
I call my Sensei "Sensei" not because I have to, it's because I want to, it's because I respect him as a teacher. The same as I repect a lecturer or a teacher in and outside the classroom. I also call people "Sensei" for those who has earned that respect, not because of the color of their belt.