Once you have a regular user base, forums can still be one of the most difficult web properties to make money with. Even with a dedicated user base, most regulars will be driven away when there are too many advertisements.
With respect to forum traffic, spamming your forum on another forum isn't the way to go. Most likely it will attract either more spammers or simply be deleted by the administrator.
That being said, it does help to promote your forum in your signature on a related forum. The same principle applies -- if your forum is about "pet care", post legitimate discussions on another "pet care" related forum, do not waste time with signature promotion of your niche forum on a completely unrelated forum -- (ex: promoting a "pet care" forum on an "airplane enthusiast" forum).
Natural search engine traffic from people searching is the best bet -- the only caveat is getting those first seeders on your forum who are willing to post and move it forward so when people do find it, it isn't empty.
I've tried RSS importing as posts, but that didn't work too well.
I think post swapping could work: ie -- "I'll post on yours, you post on mine" -- assuming both parties are genuinely interested in the subject matter.
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