I know this is done with .htaccess and mod_rewrite. This is my current code using mod_rewrite:

^(somepage|someotherpage)/?$ /$1\.php

I've heard there's a more efficient way of removing the extension. What is it?

Honestly, I found it's easier to make index.htm (or your other index file) an iframe, and have the entire website appear in it. And the actual first page of your site be something like "main.htm" and include it as a default. I don't know if this'll help or not, but it worked for me.