One of the dangers of mockups, is that they can become too realistic. Customers tend to believe that the application is “almost done” when you’re showing a realistic image of your application. Same problem for developers who think that requirements are set in stone when you give them this kind of image. Therefore don’t make your mockups too realistic. It must be clear that it’s only a temporarily design and not a final version.
Two tools that can help you creating such an “unrealistic” look: