Not sure which version of apache you're using (1.3 or 2.0), but go to the
apache website and click on the documentation for the version you use, then click on 'URL Rewriting Guide'. Then search the page for 'trailing slash'.
[edit] Or you can search for 'apache trailing slash' (no quotes) here in the forum, since this question gets asked about once a week. [/edit]
Enjoy!
--- Cerbere