What about the Rules of Art?
Beware the blanket statement. In my personal life, I get in trouble when I start making sweeping generalizations, and as an artist, I have so often been hindered in creativity by rules, maxims and principles.
I'm not talking about abandoning the professional courtesy of following guidelines for approaching galleries, submitting to exhibitions or publications. There the mistake lies in thinking ourselves above the rules - that our "gift" is of such calibre that we can remain ignorant of professionalism and artistic ethics. I am referring to our tendency to get hemmed in by the do's and don'ts of creating art - these rules are made to be broken.
Watercolor painters are told, for example, to never use black. We are expected to create rich blacks using the other colours in our palettes. While this is a rule I do follow for my nature paintings, it is not for all artists, and if you are drawn to black and can find a way to make it pop in a painting, or if you love to mix it in to grey your hues a certain way, why let a rule stop you?
Compositional rules are also more like guidelines. A strong composition will usually follow some of the principles of good design, but an artist might make a dramatic statement by having a piece that is so off-kilter it's unforgettable.
I met an artist who painted in watercolour on canvas. Not the new watercolor canvas that artists are sampling, but regular, primed oil & acrylic ready canvas. It hadn't even occurred to me to attempt that because I had been told it couldn't be done. Her results are really quite splendid.
I have hindered my work by overpainting a piece that I considered complete because I feared that "clients wouldn't understand it." I have hesitated to add other media to my paintings (gold leaf, collage, texture medium) because "juries wouldn't accept it as true watercolour." I'm a law-abiding sort and when I am given a rule to follow, it is very hard for me to ignore or break it, and this can hurt me as an artist.
By all means, be professional. That is my "rule" when it comes to marketing my art. But in the studio, when it's between you and your easel, abandon the rules and enjoy the freedom of creation.