As I have been keeping track with what I pay for, I've found that I buy more junk when I have my debit card handy. I'm going try a few tests- the first one is for 1 week I will only carry my checkbook, and then the next week I will only carry cash and see which one I use less money on.
My bet is the checkbook, cause they cost money to buy and a pain to write out. The cash is easier to throw around for me.

Later -D