WD-40 makes the rubber slide, but it also breaks down the rubber and leads to premature rot.
Do not use petroleum products on rubber unless you want to do the job again soon. I tried liquid dish soap with little success; it's too tight a channel with tight bends and the soap dried before I got the windlace all the way in. I bought a can of spray silicone and dosed the tracks repeatedly as I slid the windlace in; it slid into place with minimal effort.
