I kind-of have the opposite problem. If I have cash, it slips through my fingers. I have no idea where it goes.
When I use a credit card (and we pay it off every month), I have a trail of my purchases. I can upload into Mint and see exactly how I'm spending in each category, and track my spending that way.
I know I could write down my cash purchases in a notebook or something, if we decided to take that route. But I like being able to have the purchases automatically tracked. (And we get cash back on the credit cards.)
We have noticed recently that there are some restaurants locally that give a 10% discount if we pay cash. So that's one instance where we'll be sure to do that.