Start off with PHP-APC. That's a good start for any site. Override the cache size, and give it like 128M to play with.
Install Memcached, and use that for caching query results.
Install Wordpress's W3 Total Cache plugin, and turn everything on.
Get an Amazon S3 instance with Cloudfront, and configure it as the CDN for your wordpress site.
Configure Varnish as a reverse proxy for your Apache, but remember you'll have to
pass any requests containing a wordpress login cookie, or you'll end up with an Identity Crisis, where everyone is served logged-in user content.
That's it. That's all there really is to it. It's actually deceptively complicated, but those are the basic main steps.