WD40 + silicone lubricant spray.
I soak the all moving parts with the WD40 first. Then open close doors several times to make the boards retract and extend so the WD40 gets into everything. Then let is soak for an hour and then repeat with the silicone spay. I do this a couple of times a year. If they are really dirty like in the spring from all the road salt and crap I sometimes hose it down first to get rid of any crud. Just using a normal garden hose though, no pressure washer.
Oh when I leave it to soak I leave the running boards deployed so they dry out better.