If they still find nothing of interest, they may switch to a different search engine and repeat the process or they re-word their search. That translates to a very high amount of daily traffic to your site. Basically, there are 6 major search engines today - AltaVista, Excite, Infoseek, Lycos, HotBot, and Northern Light. Yahoo is a directory so this technique will not work with it. What you need to do is figure out what kind of pages rank well with each of these engines.
Search engines use a mathematical formula to rank all the pages in their index.
It is hard, but not impossible to come really close to knowing exactly what kind of a page will rank well on a particular engine. Basically, the text statistics on a page are what are considered most by the engine's ranking formulas. For each keyword or phrase it is necessary to create a web page that focuses on just that one keyword or phrase and ranks it highly on a particular engine. Because each engine ranks pages differently, it is necessary to create a different page optimized for each engine. No one web page can ever rank at the top on all search engines for any one keyword, let alone multiple keywords.