When starting a new forum the hardest thing to do is making it active. No matter how nice the design is, and how well you promote it, with out an active community you will have a very hard time turning visitors into members. People only want to join a forum that is active.
This may seem like a catch-22. Without members you can not have activity and without activity you will not have posters. The trick is to make your newly launched forum look active through several methods.
The first and most active poster should be yourself. If you can't be bothered to post how can you expect others to. Instead of simply posting with one username, you can post with several. It may seem underhanded but can give the illusion of activity.
Next ask your friends and colleagues to help you. Many times they share the same interest as you and can make a strong contribution. Some people have had great success launching their forum with a posting party. Invite everyone over, provide some food, drink, and entertainment, and let the posting commence.
After this you may still want to bring in more posters. Paid posting services are one possible technique. Be very careful of who you hire. Far too many paid posters are willing to do cut and paste jobs and other shay tactics. I ussually like to test them out with a small sample package.
What I find works better than paying for posts is doing post exchanges with other forums. Admin Fusion and The Admin Zone are two great places to find them. The great thing is that you can monitor what you recieve and give back posts of equal value.
By utilizing these techniques you can greatly increase the activity level of your forum. Now when visitors find your site they will find a vibrant community that when they visit they will be compelled to join.