The link is now inactive as the community has upgraded - this one helped - Use Inactivity Filters in a Smart List - Marketo Docs - Product Docs - am looking for a Specific Description of inactivity filter
Jen Farhoumand the inactivity filters tend to confuse me a bit, especially when trying to think in double negatives. What I do is create a smart list of all the people that were delivered the Summer promo email, then in my new campaignTarget Audience say "Member of Smart List Not In - Summer Promo".
Another tip: in all of my campaigns, I add those who were delivered an email into a Static List. This helps if you're going to have a complicated smart list in the future, you can just say "not in list x". Then you don't have to wait forever for the smart list to load.