There's some good advice in there, and I thought it'd be a good idea to quickly add a bit more detail about the separate approaches that Mike raises.

He goes over the APC caching, memcache, the "gzip trick", the "Not Modified" header and optimized SQL statements.

He also mentions one thing that Mike didn't mention - a split between static files (no PHP needed) and their dynamic cousins. Having a more pure Apache (no PHP installed) can help give a minute jump in speed that, depending on the size of the site, could really add up from a user's perspective.