For some reason, BMW cheaped out and didn't include door sill scuff plates for the the rear doors. For most people, with little rear passenger traffic, this may be fine. But with 2 little kids climbing into the back seat, I wanted more protection against scuffs, scratches, etc. and offer more grip than the glossy paint currently provides. So I got a roll of black "Safety Tread" tape which is about 2" wide and has a rough crinkle texture with a semi-gloss finish.
Then I cut the 2" wide tape into two 1" wide strips, rounded off the corners (to prevent peeling and also match the radius of other nearby design features), and applied them to the door sills. One strip is flat (horizontal) or parallel to the ground, and the other strip follows the gradual curve up to the wheel arch. Here are some pictures of the final result:
Some may not like the look, but I think they are better than nothing. I also have some clear 3M tread tape with a similar rough texture but I worry that the adhesive will yellow with time or get stained in this location.