One way is to get your family and friends to put in some posts. Needs to be organised so it doesn't look too contrived.
Great way to start a forum is to personally be very active, and encourage discussion amongst the people who find your site. You can also pay people to post in your forum until it gets more traffic from search engines etc.
You can make a contest for the top poster. But keep in mind the posts must have quality.. not just junk.. But that will only boost on the start... after that maybe you can offer something so people will keep posting in your forum
You need to have traffic, people will not post in an inactive forum. Seed it with basic posts and responses. Encourage participation, allow signatures, and offer a give away to top posters monthly.
There is a site called Paid Forum Posting. They can help you to get it going and get some posts in so that the forum would not look deserted.
Build a community rather than just a forum, encourage and reward participation, don't over-moderate and make sure their is a reasonable need before you start.
make about 50 usernames and talk to yourself on your forum. Makes it look active. Sounds weird but it works
1) start few initial threads on hot topics 2) ask somebody to post replies (or do it yourself with different account(s) hopefully it will make people join the conversation
Before you can get member to post at your forum, you need to post tons of quality posts to attract the people. Or you can hire some quality poster at the beginning stage. No one will like to go to forum with very less post and less active