Trickle chargers are always good, many high-end cars (Porsche, Ferrari, etc) realise their cars may not be driven on a daily basis so they often include a trickle charger. I've got a friend who's got the same one you do for his MX-5.
If your car is left outside VW offer a solar panel charger, put it on your dash and plug it in to the lighter socket, it'll keep the battery topped up (even on a cloudy grey day there's some light).
Incidentally are you with the forces mate?