I have heard several theories on this, one of which involves the xenomorph biting the inner walls of it's mouth, then spitting it the same way as a person would spit blood from a mouth wound. I don't personally buy this theory, it seems hardly a precise attack method. I would suggest that it would probably use a similar mechanism as a spitting cobra, propelling the acid from glands inside the aliens' mouth. These glands would be regularly replenished with acid, again, much like a snake's venom replenishes over time.