Besides OLP, the other smooth spin promoter I've used is pedaling at a high cadence on rollers. I've also tried that on the road, but it's not the same - too hard to find somewhere to do it continuously and be able to concentrate. Anyway, I pedal at 115-120 continuously for up to 45 minutes, once a week. I do this as a recovery drill, so keep my HR down in zone 2, the idea being to learn to do this effortlessly in a small gear. It's quite effective IME. It might be hard to hit that cadence in zone 2 right off the bat, maybe 110 and then gradually work it up higher. You could also do it at more of a working resistance, probably does the same thing.