Discounts by Category

Hello. This is me trying to figure how to setup a category discount.

I want to be able to provide during July 2009 20% discount on all items except form some in categories 7 and 11 that are new products for the fall collection. (Using ecart 4.5 and MySql and PHP.)

I want to be able to set a rule on the discounts page so only items added to the cart that aren't from those two categories get the 20% discount. Say items in catebodies 1, 2 and 3 are added to the cart as well as an item in category 7. What I want is that the discount should apply only the items from the categories 1, 2 and 3; but not to item from category 7.

Looking for some serious help here. Thanks. RP