The best solution I can think of is using server-side technology like Server Side Includes (SSI), or PHP.
I assume you are using an Apache server, if so it is likely you'll have one or the other SSI is the easier one to use.
An example of SSI command would be like
which would inset the contents of a file called sample.txt into your document. You might have to covert your .htm to .shtml to get it to work on your server.
Stefan is too fast at typing .