It's not as overwhelming as all 5 of the "W"s and the kids are usually pretty good at remembering the little saying. I WILL also emphasize that the "why" can also be "how" in some cases. We then talk about how a lot of the time, a title of a non-fiction piece of writing will often give you a broad idea of the main idea.
To practice non-fiction main idea, I cut out a bunch of old Time For Kids and Scholastic News Articles and then printed some off the internet as well. I cut all of the titles off, fold a piece of construction paper in half, and then glue the article itself on the front flap and the title on the inside flap. I paperclip the flaps together, and number each article. The kids are then charged with re-writing the title of each article to represent the main idea of the article. (I also explain that sometimes the real articles have "catchy" titles, so I let the kids write two titles if they want to--a main idea statement title, and a catchy title that encompasses the main idea). When the kids are done, they can take a look at the title on the inside and then move on to the next one.
The kids have always enjoyed this, and they often see it as a challenge to come up with a title as appropriate as possible (and then have fun coming up with a catchy one, too). I have used it for years, and by the end of the stack, the students are excellent at writing main idea statements.