If you’re not sure what a mockup (or wireframe) is… it is a basic UI design of an application or website.
For a long time, I used Adobe Photoshop for all of my mocking up needs. However, last week that changed. Now my weapon of choice is Balsamiq Mockups. This handy little tool has pretty much all of the Flex components, and allows you to quickly put together a blue print of your application.
I also need to mention that when creating mockups, don’t go too elaborate with graphics. One of my biggest pet peeves is when I spend hours thinking about and creating a mockup, only to have the graphical elements picked apart by the client, “I don’t like the colors” or “What is this Lipsum crap?”.
The “napkin drawing” (or coaster drawing if you’re a big drinker) approach is the best route to take, because it forces the clients to pay attention to what elements are there, and not what they look like.
Balsamiq Mockups… best $79 I’ve ever spent.