BPJ,
Re-volunteering. First I guess you would need to be close to an airshow unless you can get accommodation. As far as the military air shows all the volunteers that I know would be air cadets. The advantage, from the air show organisers point of view, is that the air cadets are trained and vetted to some extent.
On your own you would be an unknown quantity.
The other air show way is to find some air show participants and see if they can use a volunteer - cleaning the aircraft for instance.
Have a look at Flypast magazine that should give you some clues.