Don't wear a double breasted suit without a tie
The formality of the jacket and it being a part of a suit definitely calls for accompanying and complementary neckwear. Either a necktie or a bowtie may be worn with a double breasted suit. Since the suit is formal some form of neckwear is essential because it conveys a sense of seriousness and conservatism. Another point to consider is the fact that because the lapels cross after buttoning, your tie needn't be as wide as the lapels. A double breasted blazer needn't be worn with a tie, for example, the navy blazer with gold buttons. This one can be dressed down. However when wearing a full double breasted suit a tie is mandatory.
Don't wear a double breasted suit without vents
Unless you're making an ode to the 90s, a double breasted suit shouldn't be worn without double vents. I also have this thought of how a double breasted jacket with a single, middle vent would look like and, I can't seem to separate that thought from that of a penguin. The rules of a double breasted suit call for double vents. Vents allow for movement and their aesthetic can't be denied. For black men especially, since we have a little more derriere than men of other races, double vents accommodate the little extra we may have. The hump effect of closed vents will most certainly be avoided.
Padded jackets are so antiquated
Well...that goes for heavily padded jackets. Nowadays jackets of all kinds, single, double, etc, have narrowly set and thinly padded shoulders. The bunching on the shoulders caused by the raised arms would be diminished if there was no padding at all. Nowadays shoulder construction on jackets is done to follow the natural contours of an individual's shoulders.
Maxwell is right on one account, his jacket is buttoned throughout the whole performance. However, I feel that the buttoning rule with regards to 3 button suits also applies to double breasted suits, particularly the six-on-two jacket. Only the middle button(s) need buttoning. To strenghten the fastening, a functional inner-button, called the jigger, is usually added to parallel-fasten the over-lapped layers from the inside. Using the jigger together with the middle button will hold the jacket seamlessly in place. The top two buttons, on six-on-two jackets, are merely for decorative purposes.
No pocketsquare, no flair
A pocketsquare adds colour, flair, personality and texture to an otherwise bland and boring suit. In this case you want a pocketsquare that will have a stronger effect on the suit as most of the shirt and tie will be submerged under the suit.
These are not the final rules pertaining to double breasted suits but merely a guide on how to wear it. If the suit is a little too deep in sartorial waters then a gentleman can start with the double breasted blazer, without a tie. It is definitely a brillaint way to do casual with flair. A double breasted suit provides a special opportunity to broaden one's sartorial horizons without straying from tradition.
PG: Man to man, generation to generation.