I believe as much as 10% of your time would be well spent reviewing--and wherever possible, improving--work you've already completed and published.
At Casual Kitchen, I regularly edit and reword old posts. Once a week or so, I'll reread a few things I've written, and whenever I find something I don't like, I'll tinker with it until I do like it.
Review and reflect on what you've written in the past. What would you rephrase? Which elements of the argument or story would you cut, and which would you expand? Have you omitted all needless words? What writing aged well and what writing aged badly--and why?
Unexamined writing is wasted writing.