I think the best way is to not give anyone any reason to want to close the airfield in the first place. If there's a published noise abatement circuit, stick to it unless there's a good safety reason not to. If there isn't one, avoid overflying built up areas. Open the club bar to non-pilots and try to get the local community involved with airfield activities.
This might not stop the North Weald type of proposal where some beaurocrat wants land to build houses on and sees a potential target, but hopefully it will stop NIMBY-related issues which I would guess account for a bigger proportion of the problems.
FFF
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