This is how to do it:
1. Log into your server account. (Bluehost, Hostgator, etc).
2. Go to your .htaccess file.
For those who do not how to go to your .htaccess file, read this post.
3. Paste the custom code below onto your .htaccess file.
<IfModule mod_headers.c>
<FilesMatch "\.(js|css|xml|gz)$">
Header append Vary: Accept-Encoding
</FilesMatch>
</IfModule>
4. Then click Save Changes.<FilesMatch "\.(js|css|xml|gz)$">
Header append Vary: Accept-Encoding
</FilesMatch>
</IfModule>
Here is another tip on how to increase your page speed.