This post describes variations of an idea that reduce the vulnerability discussed in Visual Spoofing and Visual Illusions posts.
Below is an example of a phishing attempt using the visual spoofing techinque (clickthrough to see it fully). It shows a browser window containing an image of a explorer window and an fake HTML form inside a DIV section. While wary experienced users will catch on to what is going on, naive users are not likely to.

The idea of phishmarking is to introduce features to the UI that clearly distinguish the real UI from the fake UI. Appearance of the feature should vary depending on on who (user), when (time), and possibly what (site). Site dependent feature is a big topic so I'll discuss it in future posts.
When I came up with the idea of phishmarking, I was thinking about tigers, so I originally thought of using tigerprint-like patterns to be embossed into the background of UI components like toolbars and titlebars. Other patterns will work just as well as long as the pattern is not a simple geometric shape and has some random elements to it. I call these patterns phishmarks.
Note that phishmarks doesn't have to be present all the time on the UI as long as the phishmark appears briefly within the time frame that typically takes to be fooled by a fake UI. In fact, I recommend brief display of animated (to draw attention) phishmark over static phishmark which could clutter up the UI.
A simple, easily implemented, form of phishmarking is changing UI colors. But this technique is not as effective as using animated phishmarks as you can see for yourself in the following screenshot. It's same as the screenshot above except I have changed the UI theme from default blue XP theme to silver XP theme.

In this example, the color change was not drastic enough to cancel the illusion created by the fake UI and the power of branding (logo, graphics, layout). Making more drastic color changes is possible, but not without affecting aesthetics negatively. More research is needed in this area to find the right balance between protection against phishing and aesthetics.
I have other ideas related to visual spoofing and visual security and will post them in the near future under the Secure UI series. Stay tuned.
See Also: Visual Spoofing, Visual Illusions
Update:
Please read the post about PassMark patent that could affect phishmarks.