Pests in many long-running discussions--I'd say online discussions, but it is also common in the greater world of controversy--find it very easy to game the recognition of this fallacy.
The first step is come up with a superficially appealing argument that you can express in a calm and measured prose style. Put forth this argument, and spend a little time in polite discussion as your opponents pick apart the holes in it.
Then abruptly drop out of sight for a while. When you return, repeat your original argument in the identical calm, measured tone, without acknowledging that the rest of the debate ever happened.
Do this four or five times in succession. It helps if your thesis is a little unnerving, so everyone is a bit taken aback from the start.
In the later iterations, the presumption against ad hominem argument is on your side. Anyone who was around for the previous rounds will go nuts, and the reason they're going nuts will largely be that it's you again. You just put forth the same calm, measured argument as if the previous rounds never happened. Anyone new to the discussion will see a sane-sounding guy being hounded by an enraged establishment for no apparent good reason. You may even pick up some followers who will repeat your act with the sincerity that comes from being genuinely naive.
Countering this is hard. It even makes me think that, while ad hominem is a fallacy in a purely logical sense, insisting on this too strongly in an extended debate stretching over several years can be counterproductive. I think that recognition of the ad hominem fallacy was the reason the sci.physics.research charter had no provision for permanently banning a poster--and we paid for that. Oh, how we paid.