Is there a tecnique to make the sun rays that come out from the clouds more defined and easier to see. Or is that just the timing, the exposure, or just the great lighting?
I would agree - use filters. Have several handy and, when faced with a great-looking sky, try them all and keep an exposure log so you'll know which effect was from which filter. Decide which one is giving you the effect you like best. That's the best way to figure it out, really.
I think I'm a bit of a freak, because not only do I love seeing strong sunrays in images, but (most of the time) lens flare looks artsy to me. Sometimes it changes the whole mood of an image, elevating it into something more, though I appreciate it is widely viewed as a flaw or bad composure. To that I say loudly: "pfffffttttt!"