General solution
Pressing Ctrl + F5 (or Ctrl + Shift + R) to force a cache reload. I believe Macs use Cmd + Shift + R.
PHP
In PHP, you can disable the cache by setting the expiration date to a time in the past with headers:
header("Expires: Tue, 01 Jan 2000 00:00:00 GMT");
header("Last-Modified: " . gmdate("D, d M Y H:i:s") . " GMT");
header("Cache-Control: no-store, no-cache, must-revalidate, max-age=0");
header("Cache-Control: post-check=0, pre-check=0", false);
header("Pragma: no-cache");
Chrome
Chrome's cache can be disabled by opening the developer tools with F12, clicking on the gear icon in the lower right corner and selecting Disable cache in the settings dialog, like this:
Image taken from this answer.
Firefox
Type about:config into the URL bar then find the entry titled network.http.use-cache. Set this to false.