I had a job once writing encyclopedia entries for a start up online encyclopedia. This basically involved a ton of expository writing.
It was a lot harder than I expected it to be. I came to the job with a strong background in fiction, but expository writing is a completely different animal all together. In a lot of ways they are opposites. I had to hold back a lot of my natural instincts as a writer because they would have only obscured the instruction.
Honestly, my first dozen or so entries were pretty bad. It took me a while to hit my stride but once I did I was able to write in this style almost mechanically. It becomes a second nature.