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[webmasterworld.com...]
Pages added in the last couple of weeks should show up at the end of February, so just continue to work on the site, work on quality links, and give it a little time. #5 is very good, you're doing something right.
And welcome to WebmasterWorld, by the way!
Adding more content to your site is a much more productive and long term approach than sweating it out over 1 keyword.
Some months you may drop down to #40 for no apparent reason. Even then it may just be temporary. Google may be playing with their ranking algorthm.
If you drop out of the top-20 for multiple keyowrds for an extended period of time then it might be time to take another look and see if there is anything that can be done.