I think you said you were using Apache but Nginx has built-in caching and it’s simple and amazing.
Here’s a good article on it: http://reviewsignal.com/blog/2014/06/25/40-million-hits-a-day-on-wordpress-using-a-10-vps/
Basically you just cache every page for a very short amount of time so you don’t need to worry about cache invalidation. Note that this only applies to users who aren’t logged in which is usually for traffic spikes.