Shooting reflections and then inverting them is a favourite trick of mine. What's difficult, is finding the location where the reflections are interesting, having the light in the right place, and having enough stillness so that the water is relatively smooth. You can try to plan these, but it's very hit and miss. When you get a good one, like you did, it's very satisfying. I've also found that if the water is TOO still, then it's not as interesting. So having the conditions just right... you either have lots of patience or lots or luck... or both.