I have noticed that the bottom of my front door sills get very dirty in bad weather. When I get in and out of my car the back of my pant legs clean off some of the dirt. I then realized that my wife's "12 Camry did not have that problem! Upon further inspection I found that the Camry has a rubber sealing strip the length of the lower of the front door! Has any body tried to retro fit a Camry lower weatherstrip on the Prius front door?
Could be a good modification to try. C4 Corvettes had an aftermarket weatherstrip that attached to the door in an attempt to keep the sill clean. (I can't recall how well it worked) My question would be if your pants are get dirty from the horizontal/ flat portion of the sill (where this modification may work) or if you are touching the outside vertical portion of the rocker panel. Good luck....please follow up with a post and pictures etc if you try this.