"Treat everything as an experiment."  I love the idea expressed by a Portland creative consultant.  

The idea lets us loosen up and take things a little less seriously, which leads to increased creativity.  So, I like to treat design work as constantly experimenting with ideas, with volume and light and shadows, with line and texture and contrast;  all while supporting a practical program that makes the spaces friendly and intuitive.

But these things alone don't guarantee a good finished product.  The most effective design is a genuine expression of the building, it's location, history and intended use, and it's occupants.  As an authentic expression of these things, it's going to be much more successful.