Firstly, when the aircon is on make sure you do NOT have the recirc on - it keeps the moisture in the cabin and doesn't get rid of it. You need air from the outside to replace the moist air with clean air.
Secondly, make sure the drainage tube from the aircon is not blocked. If the unit cannot get rid of the fluid it creates from removing it from the air, it recirculates it regardless of the setting.
Finally, check you don't have a leak somewhere in the boot. Generally I would leave the A/C on all the time, on auto and simply move the temperature gauge as you need to.