giving them back links. (signature links) Top few get (say 20 top posters) every month get site wide link) Getting good rank for your forum (buy lots of links and show you are serious) Make sure users think, you are going to stay not just gone in few months. etc..
This works. Works best with users that are webmasters or website owners. Great tactic for webmaster forums. However, the forums I have in mind are CONSUMER forums. Same critique as above. Only webmasters would be interested in this. I'm trying to get CONSUMERS to post. This makes sense. My army of writers can produce "pillar articles" for linkbaiting and resource purposes. Any other suggestions. Keep the good stuff coming!
Good content and something to actually talk about. Also offering incentives to post such as a higher member rank based upon their knowledge of the subject can help too.
Give free stuff (anything from T-shirts to money) to top posters. Come up with contraditive ideas and ask for opinions (like... in Linux forums throw a post which leads to ultimate fight over linux versus windows - apple is naturally forgotten ) Arrange a some sort of contest. Email your forum users (don't do this often) and remind them about some new stuff.