The rule that I like to use is if it's $10 or $20, pay cash for it. If you can eat it or drink it, pay cash for it. And the best example of that is reported in the Wall Street Journal. When McDonald's started to allow people to use credit cards instead of paying cash, the average sale went from $4.75 to $7. Need I say more?
Hmm, the implication is that it might be the small things that add up, rather than the big ticket items. I have been using my Citibank Dividends card to accrue cashback for my monthly expenses, and always pay my balance in full, but I wonder if I should go on a one-month trial paying cash instead(for everything - grocery store included) to see if my expenditure drops.