What color are the leather seats? If they're a light color, you may run into a problem with dye transfer. I have tan leather seats, and the black from my denim pants and from my leather vest eventually works its way into the leather. I've tried dye removers, but they were too aggressive and left spotty areas.
Right now I cover the drivers seat with two white towels. I also use Maguire's leather cleaner every couple of months to remove any dye that may have transferred. It's a PITA, but the seats look good.