how to decrease serverload easily?

We're developing a web platform based on PHP, running under Apache. We have realized that we need a way of "split up" the serverload to multiple machines if it gets too high. We've come up with PHP solutions to the problem but are looking for better ones, especially in Apache configuration. Is there any easy way of doing this? Anyone have previous experience? Software versions and OS'es is of no concern, we'll use what's required. /Martin