Keep it simple: In general, interfaces should use simple geometric forms, minimal contours, and a restricted color palette comprised primarily of less-saturated or neutral colors balanced with a few high contrast accent colors that emphasize important information. Typography should not vary widely in an interface.

A powerful tool in the early stages of developing scenarios is to pretend the interface is magic. If your persona has goals and the product has magical powers to meet them, how simple could the interaction be? This kind of thinking is useful to help designers look outside the box.

No matter how beautiful, no matter how cool your interface, it would be better if there were less of it.

Define what the product will do before you design how the product will do it.

Form follows function straight to hell.

Well madam, have you looked in the mirror and seen the state of your nose? Boxing is my excuse. What's yours?

Ironically, the thing that will likely make the least improvement in the ease of use of software-based products is new technology. There is little difference technically between a complicated, confusing program and a simple, fun, and powerful product.