Doing Mental Math when I Grocery Shop Never Works!

As usual, I went grocery shopping today without a list, and I was very limited in time.  I am sure that I would save so much more money if I would myself like the folks in "extreme Couponing" but that just isn't the case.  Besides the fact that I know I wasting money, time, I am also  buying food and items that I really do not need.  My point in all this is that I justify my actions by all my waste in that I use math in my head to do calculations, but in the end its just not very organised and efficient.  There is a time and place for mental math- and its not at the grocery store when I am in a rush!  I often find myself making quick decisions, and trying to add items in my head as I move throughout the store but I have finally realized that it is a waste of time.  When I checkout, the total is always so much more than I expected and I always promise myself I will make a list and bring a calculator next time. Another thing is adding the tax.  I always mess that up to.  I really do need a calculator, coupons, time, and a list next time I go shopping!