cgrantski - 12:11 pm on May 19, 2009 (gmt 0)

>>> the rank varies quite a bit even for keywords where I've had stable ranking for years. <<<

That's probably because although it seems stable to you (checking from the same computer all the time) others see quite different search results, depending on their previous browsing and searching activity. On Google, anyway. Where I work, we like to show the client a trick where we can force a given site from the #10 to the #1 position within just a few seconds, by doing the right click-browse behavior immediately before the search where we want the target site to jump to the top position. So, if you think you are #1, but some people see you as #2 sometimes, it might be because they had been doing click behaviors that jumped one of your competitors into the #1 position, pushing you down to #2 for that particular search.

Another reason could be geography. If one of your competitors is more appropriate for a given searcher's geographic location, they might be shown as #1, above you.

A good custom report to look at the geography question is a a three-tier drilldown --- keyword, rank, geographic location. Then you could see which geography is working against you, for which keywords.