You can buy these (price around as this was just chosen for the photo)
http://store.rvdirect.com/rvdirect/d...p?ITEM=4410051 or make them from something you have around the house. They're basically a plastic runner for both ends of the slide. Even if the slide is in perfect alignment, over time the carpet gets worn and darkened where the ends of the slideouts run. For flush slides, be sure you let the slide come up the ramp a couple of inches before you place the slideout slickers in position. Like earlier mentioned ... carpet cleaner gets rid of the black marks ... rinse well as soap left behind attracts dirt quicker.