Use soft words and hard arguments.
Proverb

A forum (or bulletin board) allows free-form discussions between your staff and your visitors. These work best where there is an established community of like-minded individuals with an interest in a particular topic – connected to your business. If such a community doesn’t already exist then starting a forum may be a risk – an empty forum doesn’t reflect well on your business.

But if you can gather a community of users to your forum you will benefit from a constant flow of new content that keeps bringing people back to your web-site.

Preferred applications:

SMF
Vanilla

Others to consider:

vBulletin, phpBB, punBB