Personalization and Customization.
If you really want to be generous then unlike some free forums, you will leave the Style chooser in place and make sure that members know about it, this way they can vary between what suits them best. As some skins, such as SubSilver are harder to navigate around than something like ProSilver, etc. And you'll find that most styles are based on the newer, sleeker looking style rather than going backwards. Some members may want something like out of this world Graphics design, while others, a quick loader. It varies person-to-person, so keep options open. Also, find the most popular skins and then disable the rest that aren't in use (if any), because this helps establish a site by varying your skins, however, keeping it small.
There are a good range of skins available on PhpBB3now already, and I'm sure they'll add more in future. So, please, just keep options open for members, allow them some personalization.
There are various ways of advertising, such as the aforementioned affiliation, however the biggest one (for some) would be word of mouth. If you are around a few mates who like forums, then start telling them, hopefully they'll pass it on, and the process repeats itself. However, you have to watch your audience of who you’re spreading the word of mouth too; some people do not like forums, so watch it.
One method of advertising that you should never do is post links on another forum and spams that forum with links, upon links, upon links. It gets annoying and you could be banned for it.
Also, when word of mouth is put into action, make sure that it’s something that attracts an audience, something new, something with a buzz!
With Forumotion, you can participate in their forum directory, that will give an unique way to advertise quickly and efficiently free forums!
Don't start too big
Everyone knows, “Slow and steady wins the race,” however, usually this doesn't work in normal races, but it couldn't be truer in free forums. However, you do need to take risks; some people pay for advertisements, try getting the forum out there through Google, and actually make the forum look active. By this I do not mean sign up with multiple accounts on your own, just get a few friends to join and post, and make the forum look active. This attracts.
Just don't overdo it on advertisements, and think that your forum will grow right away. Try just getting it out there.