On the blog Scalable Web Architectures there is a new post called "What is Scalability?" I wanted to point out that it is definitely a good read and worth the time spent. In summary, it's a detailed explanation of several of the terms and concepts that encompass scalabilty. Misconceptions persist about what scalability really means and this article can help dispell some of those misconceptions. Most people seem to get it wrong and confuse it with performance in my experience.
One part that I would like to highlight before sending you on your way is the comment Royan's makes that, "there is no one size fit's all solution." So very true.
There are definitely best practices for ensuring scalability. But, there is no scalability silver bullet that will always work for every situation. This is especially true when an application has already been written and then someone says quietly, "but... will it scale?" The closest thing is, as noted in the article, buy a bigger box. That will indeed take you a bit further but only to a point.