The Carnival drink packages never go on sale. The only way to get discounts on Carnival drink packages is to book it before you cruise. This will give you a $5 per day saving on the Cheers package compared to the price you will pay if you wait until you're on the ship to buy it.
Carnival rarely offers a sale or promotion on the Cheers package. They will take 10% off the cost if you purchase it before you get on board, but otherwise, the package doesn't usually see any discounts.