The search engines send out their crawler every couple of weeks. This will naturally change your listings a bit as the engines find new sites or lower the ranking on existing ones.
However, ever 3-4 months the engines update their algorithms and everything tends to get turned on its head. Yahoo Just recently had an algo update but all in all if you have good SEO your sites will need some adjustment to get back on top after these changes.
Or you can just come and sign up to my program and we'll do it all for you
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