Selectable Templates

I wanted to have the ability for a user to use two different templates, not just style sheets, when using WordPress for different purposes. So I messed about a bit with the index.php file to change things, and thought I’d create another content layout called “main.php” that showed things differently. (I know, it’s an ugly hack.) But every single link reverted bag to “index.php” as the reference as soon at it was used. Maybe I overlooked something simple here.
Is anyone using template files with WordPress? Any examples I could learn from?
Thanks,
PEB