Howto output HTML from drupal module the nice way

this may be basic, but i just started using Drupal (6 btw).

I'm building a module, and got a callback that is supposed to return some html. I could just do something like this:

function myModule_myFunction(){
    $r = '';
    $r .= '<h1>'.$variable.'</h1>';
    return $r;
} 

But I'd rather seperate logic and presentation, and put this in a seperate file, or something. HTML in strings is ugly!

What's the 'drupal' way to do this? It doesn't have to use the theme system as far as I'm concerned.