Just deactivate and activate it again and the 6 month clock resets. This is the easiest way. Don't bother with running fake people through the flow just to keep it live, because once they run through the 6 month clock will begin again until another person runs through.
You also get a warning of which campaigns will be deactivated, so you only really need to deactivate and reactivate during this warning period and the 6 month clock resets.
And of course, vote for the idea: Trigger Campaign Deactivation Maintenance
I'm curious if the Marketo Sky roll-out will have a better way to do this en mas. You'll be able to deactivate and re-activate many campaigns at once, I'm just not sure if you can filter them in a way that would flag them. Possible use-case would be to include a key phrase in your naming convention "-keep-on-" then you can filter by those and either turn them off/on before they deactivate, or simply go back after they deactivate and see if any were turned off. Ultimately the use-case of them being inactive for 6 months should be a trigger for you to do some better promotions or discover reasons why there's no activity...