The World Naked Bike Ride coming to / happening in a city I believe typically goes through several distinct phases over the course of years. This was true for Chicago, and I gather from reading was similar in others. I mention this so new rides can address appropriate needs and expectations.

First year and before is usually championed by an individual, and a few dear cycling friends. This is MUCH easier if a Critical Mass monthly ride already exists. If it doesn't, that should be established as well, perhaps first. Champions probably are notably comfortable with their bodies, and local nudists may aid. First year may simply be flyer printing, word-of-mouth, and under a dozen turnout. BUT AT LEAST precedence is established.

Second and third years are about familiarizing the city and cycling community with that the event will happen, and has happened before. City becomes familiar with the name. Helps if prior year photos are posted and good painting was done. Do heavier flyer printing, word of mouth, and PR. For rides under a hundred, route can be informal and start/end as simple as someone's yard.

By third year, cyclists+hipsters will expects the event in advance. Press will readily publish it in event listings and may ask for photos. Expect several times the prior years riderships; in smaller cities be prepared for dozens, larger should hit hundreds with proper publicity. At this level a planned route is required, start/end points become significant questions, and legal observers and police liaison should be engaged.

By fourth year the event is a serious sustained production, where all the matter in this How To comes to bear, and a considerable team of facilitators is required.
As of fifth year Chicago ceased pursuing increased ridership. With the mass long enough to fill the biggest streets for miles, we have the luxury of concentrating on safety and message, and the new challenges of such a colossus.

Chicago has been notably successful due to the current good level of organization built through experience. BUT another key to success is that work is appropriate to development year. Chicago currently pursues an optimized balance of maximized safety, message, fun and comfort, with minimized friction, management and cost. A first few years ride however could simplify down to get lots of riders, have fun, and don't get arrested.