I've joined many forums that were focused on the same niche as a forum that I owned. Every forum has it's own style/voice/flavor. For my part, I just participated as normal and did not link to my site in my posts or signature (unless it was encouraged and something everyone was doing). I would include a link to my forum as my homepage in my profile if the forum allowed it. Other than that, I'd engage in topics that interested me.
Then again, I once joined a forum in this fashion and got an email from the owner a few days after registration informing me that I was banned for 1 year because the owner noticed that I owned a competing forum. It was the dumbest ban ever. I remembered about the site a few years later and started participating as normal infrequently. About 10 years after I had first joined, the owner of this forum decided to pull the plug and some of the community there migrated over to my site.