There's a new weapon in the search engine wars, and it's known as AnalogX Keyword Extractor (KeyEx)! KeyEx gives you the ability to look at your web- site much in the same way that a search engine does, with customization to adjust the results to match the subtle differences between engines as much as possible.
The actual operation on KeyEx is very simple, just load a file from your local system or even from another website (by simply entering the URL). Once the HTML is loaded the program then analyzed the contents and sorts the results based off the search engine profile that's currently active. The important value to watch is the 'Weight' value, which is the relative strength the search term will probably have at a given search engine - the profile can easily be changed by just selecting Profile->Add/Edit/Remove and selecting which engine you would like to evaluate as.
The current profiles are just rough approximations of what the different engines evaluate - you can modify these values to help dial it in to whatever yields the best results. The weighting values are multipliers used when the actual evaluation is performed, so if you have a weighting for BODY set to 0.5, it will multiple the number of occurrences of a keyword in the BODY; so if the keyword count in the BODY is 8, then the weighting value would be 4.
So how do you use the results? Simple, take a look at some of the pages that rank higher than your page in the search engine - then load them into KeyEx. Now, look for which terms are ranked higher on all the other pages that are listed in the search results - those are the keywords you'll want to use in much the same way as they are in the other pages. One important point is that some sites run special programs that give different results to search engines than uses, so if you find the results are radically different than many of the other results, than that's what's probably happening.