I've been wondering about this for a while and found an article with an interesting take on it.
Basically write testable code. Don't code for possible future reuse. Only make something reusable when you write a new piece of functionality and notice it is similiar to something you did before then make it reusable.
I like this because I see people spending a lot of time architecting reusable solutions when you can tell it will never be reused. However, when reuse makes sense, you can still make use of it.
http://agile.dzone.com/news/reuse-myth-can-you-afford
