One really great way to reduce the amount of 404 hits on your site is to have a "full text Search" as your 404 page, this is also really great for SEO (Search Engine Optimization)

You can enable this option in your "Site Template and Settings" area.

When using this option you can have the system search, instead of showing a 404 page.  So for example if you had an alias "christmas" indexed:  http://www.YourDomain.com/christmas
And that URL no longer exists.  Instead of a 404 error page showing, it redirects to the "Full Text Search" page and searches for whatever characters are used after "http://www.YourDomain.com/" so it goes to that page and searches all pages and products for "christmas".  This is also helpful if people had an old website and now all of the page and product URLs are different.

NOTE: if you select the "FULL TEXT SEARCH" in there are no pages for you to select in the second drop down, this means you need to add a page with the "Full Text Search Result" frame and the "Full Text Search Box" frame.

see screenshot:

Another cause of increased 404 errors on your site is altering a category name.

Categories create automatic aliases for a "Category page" for example: http://manual.websiteforge.com/5m2/ways-to-get-help.html ("ways to get help" is a category)  Since that category is already indexed on the Search Engines, if I make a change to the category name in any way (even changing a space  or adding 1 character) the URL will no longer work, because the alias name will not be the same anymore.  Having the full text search as the 404 will help eliminate this problem as well.