Search engines scan the Internet constantly, and as new sites become indexed, others drop off the lists. Monthly submissions will ensure that your site remains in the indexes for the various search engines.

Also, just as out-of-date sites are a turn off for visitors, they are a turn off for many search engines as well. A number of search engines can determine how often the site has been updated and factor that into the rankings. A site with frequent updates will receive a higher ranking (and therefore more visitors) than a site that does not update routinely.

Some search engines solicit payment to "guarantee" top placement, but this is not necessary. Good website design, the creation of good keywords, regular search engine submissions and a little patience usually ensure adequate placement in most of the search engines.

The one exception I would make to this would be the Yahoo directory. Since so much potential traffic could originate from the Yahoo website, it would be well worth the $299 submission fee they require.