I hate what forums consider advertising, actually. They're not really ads, per se, but rather link exchanges which at best have diminishing returns on your investment (the more time you spend exchanging links > the more your ad appears as people search through ads on forums > the less impact your ad has as people are more inclined to pass over repetitive ads).
Link exchanges are, however, good for SEO, as exchanging your link on sites that get frequently crawled and are therefore considered "legitimate" increases the legitimacy of your own forum (which, coincidently, is why spammers do what they do). So the trick is to find a way that cuts down on just effectively becoming a spammer by flooding the market, and figure out what your demographic is.
Ultimately, I prefer a much tighter, more organic way of advertising: word of mouth and affiliates with forums that specifically attract the kind of members I want to attract.
1. Word of Mouth: the best advertising is getting people to recommend it to their friends and to become advocates of your brand (so to speak). So, a bulk of my focus is cultivating an excellent user experience (whatever that may entail), and letting the members be our ambassadors. You're more likely to get genuine interest in this way, as people are social creatures and tend to be inclined to invest in communities with good atmospheres where they know at least one person.
2. Selective Affiliates: I don't affiliate with every site. Rather I affiliate with sites that a) do not compete with me and b) attract they type of audience I want to attract. So for instance, for a fandom-based RP, I affiliate with the fandom wiki site, a screencap/resource fansite, fandom news sites, and fandom social media. I also affiliate with fan sites of similar genres (so if I run, say, a LOTR site, I would affiliate with similar, but non-competing forums like GoT, Elder Scrolls, etc). Lastly, I find one site to be a "sister" site that is the same nature as my forum, but with a different premise. Going back to the example, let's say my LOTR forum is RP, strictly canon set in the events of the Fellowship of the Ring. I might sister site with a forum RP with a modern AU setting, or set in the ages before or after the main series.
For me, this cuts down on the repetitiveness and sees a higher return on investment, and gets people who are more willing to long-term engage with my forum and community.
Of course, your mileage may vary