Mine does this, not too bad now after cleaning the IACV.
First things first, buy a replacement gasket. You can pick them up for a few quid.
Take a picture of the IACV before unscrewing so you can get the screws back in the same position, otherwise it could alter your idle.
Unscrew and unplug the IACV, take some petrol or carb cleaner to the valve and you can let it soak for a while. I used some cotton buds to remove the heavy build up of carbon.
Once it's cleaned and dried, put your gasket on then reattach!
Pretty straight forward :D