I use a C-TEK charger which is a battery maintenace charger rather than a pure trickle charger.
C-TEK also produce a lead with a cigarette lighter plug on the end so that it can be connected to the car via the 12 volt socket under the dash. (This socket is permanently "on" and not reliant on the ignition being "on" and therefore is the most convenient way of connecting the charger to the car).