I just took my first go at it today. I'm not sure if i've got it completely down but I assumed your first trip you may have to spend a little money.
Today I went and spent $51. I bought things that I needed that also offered ECBs (a few of which I had coupons for from my local paper.) After using my CVS card, I ended up getting right at $30 ECBs coupons.
So my next trip, I'll do the same thing (buy items that offer ECBs) but I will use my ECBs to pay for them. So I will have to pay next to nothing plus I will get more ECBs to spend on the next trip. Does that make sense?