Sketching for Concepts and Initial Wireframes

A fast way to deliver concepts is through sketches. In my experience you can spend lots of time faffing about with power-point or visio to achieve something perhaps half as good as you can achieve with a sketch.

For impact you can go for color and quality:

Equally, to explore layouts you can produce something quickly that is good enough for your stakeholders to review:

We can't claim that we developed this way of working though, we used Isaac Pinnock from MadeByMany who works in this way, some examples of his work include the Telegraph Blogs

I think the speed with which sketching allows you to progress your ideas fits well with agile development. I have found it useful when I have created a user story and want to give design more than just requirements to go on, this alone is probably where you can gain time in iterations. Let's be honest it's also a refreshing change from burying your head in a computer.