Oh Drat! dasilvarsa has come up with a good idea. I suppose we'll have to be serious again Giomen. Following on from G's alcohol breath idea, it's actually true that cleaning the glass with metho or an alcohol based cleaner such as Windex and then thoroughly polishing the area with newspaper (yes, newspaper) before applying a silicone based dash product works extremely well.

If it seems like too much trouble then just use spit ...