Sort by topic, rather than client. If some of your article topics are similar, write those sequentially or at the same time. Save them to separate folders for each client so you don't lose track. But you can do articles for various clients all at once. This is especially helpful if their deadlines are the same or close.
Do all your research first. Research your topics before writing. This makes it easier to just sit down and write. You get bonus points if you took notes in your article document during research. Medical info and other extensive topics can require a good amount of research for accuracy. Bullet point what you discover under subheadings. Then, when you write the article, you only need to turn those points into sentences.
Group by article type. If you have several list-type articles to do, it can help to do those first. Those may be easier than other formats. Articles with bullet or numbered points can go quickly as well. You also may wish to group them according to word count. When large numbers of articles are due quickly, it's a better strategy to get all the easiest ones out of the way first. This way, if you do fall behind your goal, it happens with less articles.
Write intros and subheadings first. Go through all your article files ahead of time and write all the intro paragraphs and subheadings. This way, you can run through and fill them in faster. If you already have the subheadings labeled, you pretty much know the points you need to make. You then only need to figure out how you're going to say it. This speeds productivity trick. I've written an article with this type of pre-outline in less than 3 minutes.
Write what you know. If the topics are up to you, avoid too much extra research and just write about what you already know. When writing topics you have firsthand knowledge of, it's easy to just spout off info quickly. This is extremely beneficial if you're also a fast typist. It may take a touch longer if you aren't, but it should still help considerably, compared to articles that require extensive research.
*I originally published a version of this via Yahoo Contributor Network