Depends on the size of the dragon
If their heads are bigger than a person I wouldn't put it past people to simply ride on the necks just behind the heads as some sort of 'ride by the horns' method.
If not then maybe somewhere just forward of the shoulders as a sort of horizontal piggybacking method with reigns like how you'd control a horse.
Either way a lot of dragon riding as depicted to be on the back would actually very greatly mess with the back muscles controlling the wings and then you have to file down dorsal spines or inhibit vertical fins and various other protrusions typical of draconic creatures that usually aid in the control of their flight through the air. Really, on the back just isn't that great.
Could also simply be in a basket carried by the dragon by their talons, 'riding' them in a similar manner as you would use a hot air balloon, but that's so far away from the 'back riding' direction that you want to go to be on the ridiculous side.