Pick the smallest topic that you can think of, or something that you know literally nothing about, and write an article about it without doing very minimal valuable research. I started Major urinary proteins like this. This works out the limits of your creativity and imagination.
Pick a subject that you genuinely love, or are fascinated with, and write all that you can about it. Too often people fall back on negativity to carry humor across, while this trains you on pulling funny situations out of positive moments and sincere appreciation. I do this all the time: Charles Mingus, The Wire, Talk Talk et al.
Collaborate on an article with two or more other users. When adapting or complementing other people's styles into a single whole, you gain invaluable perspective on both your own writing and the idiosyncrasies of others. See: Uh oh!, HowTo:Be Deep, and for a big time example of this, Inbox.