First rule you should never break. GET AN EDITOR! I know they cost money, but it's well worth every penny.
I broke the "Do not get your friends to design the cover" rule. Excuse me, the people that came up with that rule had good intentions. They assumed that you would find some crafty person to make your cover. Well, my husband is an artist. And I personally know at least four professional graphic artists. If you have that sort of connections, why not use them.
So my husband designed the cover. We sat down and discussed the color layout, how we wanted to make each cover in the series related to each other. And how to make the cover stand out on the computer screen and on the shelf.
I am very happy with the work. And with the latest typo meltdown, his artwork must be good, BECAUSE NO ONE SAW THE MISTAKE FOR MONTHS! In fact, I was the one that discovered it on the cover, after my friend pointed out the mistake on the sell page.
A rule I would like to add: Find a off-line place to sell your books. Not just a regular bookstore. Think outside the box. There are other places that sell books, that are not "bookstores". Mine is going into a art/metaphysical type of store. If you make a cookbook, see if an independent grocery store will sell it. If you write horror, look for a costume shop. The internet is fine, but most of the people that want a copy, are people I have actually met. From them I hope the interest grows.
Any other rules worth breaking or adding.