How to build for scalability

Maybe you were thinking this was going to be a tutorial or a methodology for good practice, but it’s actually quite the opposite. I agree that scalability is extremely important, but I don’t exactly know what it means to “build for scalability.” Well, I know what it means, or at least the point is made, but how do you do that? What would I want to do differently? I’ve read a number of times that you should “worry about your users once you get them; just set your app up for scalability.” It seems like it’s a hardware issue, but I’m sure it’s more than that. Maybe by building a service-oriented application, you can eventually host each “service” on a separate, dedicated machine to handle large amounts of traffic? I’ve looked it up and read a little bit about it, but nothing jumps out at me to claim that I know how to do it. Surely it’s more than optimizing SQL queries. Luckily for me I don’t have (nor am I involved with) an application that requires sound scalability.