I know that it's extremely rare for me to post about PHP... but hear me out.
This does not make for an awesome start to a preview + status call.
Bust Those Caches
The easiest and quickest (and dirtiest?) solution I've found to remedy this issue is to use a PHP-driven UTC timestamp URL parameter cache buster on files which tend to be cached by my clients' computers. Check it:
<?php echo time(); ?> will barf out a UTC timestamp, ensuring the URL is always "unique" (enough) to bypass the browser's cache and always pull the latest version.
The parameter name
r bears no significance here - in this example it stands for "random," but you could name that
/?fhqwhgads= if you really wanted to.
You might not need this for all your clients - but it tends to be extremely useful for clients who might not know how to clear their browser cache.