The other day our servers were overwhelmed with the rising traffic and I started profiling my application through Xdebug.[...] On this project we currently run with 100,000 visitors per day on average, our peak is Sunday night where we get a ton more traffic than usually. [...] Here are a few things, that helped us. Suggestions are in no particular order and I should add that whatever is applicable for my situation, doesn't have to work for you.
His suggestions include using APC for caching, using __autoload, caching of DB queries, and some Apache tweaks to reduce what it has to load/use for each request.