If you plan on using Amazon S3, sign up for an account at https://aws-portal.amazon.com/gp/aws/developer/subscription/index.html.
Create a bucket for your files. Pick a unique name such as
assets.mysite
.Using your site's WHM control panel (or edit the DNS Settings file for the site on your VPS), create a CNAME record for this bucket called s3, and point it to
assets.mysite.s3.amazonaws.com
. (the period at the end of this is important).If you aren't planning on using Amazon S3, then instead of using a CNAME record like the one in step 3, just create some subdomains called assets1, assets2, and so on. You can then create those subdomains as VirtualHosts in Apache.
Upload popular theme files (your logo, nav buttons, css files, and so on) either to Amazon S3 or to those subdomains.
Edit your theme files to reflect the change in path to the assets. If you're using different subdomains, you could have images on assets1, JavaScript on assets2, and so on.
That's...