This is all useful information, and very helpful to anyone that is starting their very own forum. It took me a little bit to read through the 6 pages so if I repeat any of the information that has already been provided I want to apologize for that.
One of the big things to always remember with creating a forum is like it has been said the content inside the forum. I'm a moderator and member at several forums, and I have noticed that you need the forum to be based off of one specific genre. For example if you want your forum to be a gaming forum then make it mostly a gaming board. Maybe have some general things in there. If your wanting it to be just a general board with different topics; that is where it can be a bit of a problem getting members from what I have noticed, because there is a lot of boards that are just general things.
Now lets use my board as an example. My board is a general forum board, but also is a gaming board for all gamers, and for my gaming clan. I've combined 2 different things into one board. Now the way I have my board set up could make it a little more difficult to get members, but I'm making mine to where pretty much everyone could become interested in it.
Modifications is another area that could be a problem for members or guests that might want to become members. Having to many modifications could confuse members. For example if you have the SMF Shop, trade, arcade, and any other different addons like that could make a member not know where to go.
Granted customization is very important to a board so it is different from other boards, you will need to keep it somewhat basic, but of course make sure it is customized.
Advertising your site through your signature on other boards like what was said earlier in this topic can be very important, but you don't want to make it to obvious that you are wanting people to join a membership at your board. Maybe create a image or use a software that will allow you to take a snapshot of like your banner, post that in your signature, and make that image a clickable link if the board will allow you to do that.
I hope this added information was some good information to members here to help get their board going. Once again I am sorry if any of the information was repeated as it took a while to read everything, and to remember everything that was already said.
P.S. one other good way to also rack up some membership is to maybe have a actual website page with different things in it, and then have in there a link to your board...