Figuring out which keywords will make the most money for residual income articles is difficult. The best way I've found to test out ideas is to use a site that caters to short articles. I used to use eHow for this, but now that their writing program has shut down I've moved on to using List My 5.
My earnings there have been comparable, plus the site is laid out a lot like the old eHow. These articles take about 10 minutes to write, tops, so they are quick. Very little effort to see which of my ideas are hits and misses. Once I know, I use the subjects and keywords in articles for my blogs, websites and Suite 101 articles. I can almost always replicate my results in the longer articles. Give it a try and let me know how it works for you.