When I'm planning an article, I find it useful to write a short sentence that I think encapsulates what the article is about. As I'm planning and writing the article, I very often refer back to that one sentence to ensure that I'm staying on track.
Sometimes, if I wonder off the track, I make a note of where that wandering has taken me so that I can use it for a future article. That way nothing is wasted. I also think about why I have wandered off track: maybe the article brief I've designed isn't wide enough and it needs to be expanded, or maybe it's too broad and I need to rein it in.
Spending a little time thinking about the remit of your article can save you a lot of time and heartache later. If it's flexible enough, there's no reason why you can't change and adapt it as you go along. Perhaps, by the end of it, your remit might have changed completely - that's the beauty and frustration of creative writing!
Always keep in mind what it is you're writing about and what you want to say.