At work we're experiencing some fairly rapid growth, and our single production server is starting the feel the strain. I've been doing a lot of investigation into how we can scale the site, and thankfully there is lots of information out there.
The "Do you Scale" presentation I saw at PHP London a couple of months ago gave a good high level overview of scalability issues, and included some useful techniques to help you scale.
I think I've found the ultimate scalability presentation though: "Real World Web: Performance & Scalability". The 189 slides contained within this presentation cover almost everything I've read elsewhere, and it's packed full of practice advice!