You don't need to use the traditional *.shtm and *.shtml filename extensions. You can ask your ISP to turn on SSIs for your entire site. Or you may be able to do it yourself without his permission by inserting a file named .htaccess in your root directory containing such lines as:

Options Includes FollowSymLinks
AddType text/x-server-parsed-html .html
AddType text/x-server-parsed-html .htm
AddType text/x-server-parsed-cgi .cgi

SSIs place an increased burden on the Web server, since now it must "parse" or carefully scan each *.htm Web page is it is called to find and deliver its component elements in order to display the Web page. Thus, ISPs don't really like SSIs.
If you use FrontPage 98, you might try the "Include Page Component" which does the same thing as SSIs.

SSIs are thing a beauty, but not for beginners. The documentation available on Server Side Includes is poor. Yes, all the elements are there, but they seem pretty cryptic when you don't know what they mean. You'll just have to do some trial and error to learn this, though you'll find some help with these resources:

• Mark West's Server Side Includes Tutorial offers lots of guidance as well as e-mail help. http://www.carleton.ca/~dmcfet/html/ssi.html
• NCSA Httpd Tutorial: Server Side Includes (SSI) http://hoohoo.ncsa.uiuc.edu/docs/tutorials/includes.html
• Ben Laurie and Peter Laurie, "Apache: Server-side Includes," Web Review, October 10, 1997, excerpted from Apache: The Definitive Guide (O'Reilly, 1997) http://webreview.com/wr/pub/97/10/10/bookshelf/index.html

Begin by testing a file that contains just one or two of these commands before you set up an entire system.
If your Web page comes up with the message "[an error occurred while processing this directive]" that indicates that the server is scanning your page for SSI commands, but something is wrong with the command within your Web page. If you see this error message, your .htaccess files is probably working okay.
Look for picky little things such as a beginning forward slash in front of the virtual command

/ssi/filename.txt.

Also, some servers or some CGI programs running on SSI pages may require you to remove any spaces between the actual command and the outer "envelope." Make sure you reload/refresh the Web page with your browser after making these tiny changes. Otherwise, you may have fixed it and not know it.
For further help consult Mark West's Server Side Includes Tutorial.