Make sure that AllowOverride is set to an appropriate value. Then, to make sure that the .htaccess file is being parsed at all, put the following line in the file and ensure that it causes a server error page to show up in your browser:
Garbage Goes Here
.htaccess files override the settings in the main server configuration file. Because this is frequently an undesired thing, .htaccess files are frequently disabled, which will cause your .htaccess file to be ignored.
.htaccess files are enabled using the AllowOverride directive, which lists categories of directives that may appear in an .htaccess file. For example, if you wish to put authentication-related directives in an .htaccess file, you will need to put the following line in the main server configuration file:
AllowOverride All permits any directive to be put in the .htaccess file, while the directive AllowOverride None means, "Please ignore my .htaccess files."
Thus, the most common cause of an .htaccess file being ignored is simply that your configuration file tells Apache to ignore it.
If you put garbage in your .htaccess file, this should generate a Server Error message in the browser, which will verify that Apache is indeed looking at the contents of your file. However, if such a message is not displayed, this is a sure sign that your .htaccess file is being completely ignored.