That is the first thing that you should do. Send letter to organisations in the field and let them know which areas are being covered and how they can benefit from the workshop. If they want to send a group of employees, do offer them a good package deal to make it even more interesting. You can also contact individuals in these organisations if the company itself is not interested.
There are companies available who will send tens of thousands of emails out to subscribers about any deals or special offers. These are voluntary subscribers and your emails will not go to bulk mail. You can choose the market segment you wish to target and send the emails out to those particular people only so that you do not waste resources.
Make a website of the event. You will need a static page that provides all the necessary information and in case that there is any change, you can update it. You can also setup pages on various social networking websites and ask friends to promote the event. Keep ways of communicating back to you open so that if someone is interested, they can get in touch with you right away.
You can have carefully placed hoardings in areas where the corporate offices of businesses that are related to your workshop are placed. Have a small but effective hoarding that is visual. It should do well as the word of mouth spreads pretty quickly.