By Terrorz of AdminBB.org
1) Patience: Its a key factor in making a forum, and if you don't have any, you might as well not start, because your forum wont go anywhere.
2) Passion: You have to have a passion for making a website, and love it. You cant just make a forum because everyone has one. You have to be really be involved in it for it to be successful.
3) Advertising: You have to have some knowledge on how to get members interested to visit/join your site, or you can just be the only member. There are many websites out there to help you in your goal to make a successful community. Thats one reason why AdminBB.org is here, to help all admins like you reach your goals.
4) Supply & Demand: You have to provide something on your forum that will get people interested and keep them coming back for more. Like ForumPromotion.net is a great example, webmasters visit it frequently on hopes of making their forums more successful, and it does help.
5) Friendliness: You have to be friendly in order to have happy members, and to make your forum have a good atmosphere. Don't be to strict on people for not following your rules. Let them know in a nice manner, and they will gladly listen to you. If you send them a infraction for something small, you are just going to loose another member.