Everytime I go into a store I always have coupons. To make sure that I know what I need to get and to keep the coupons handy I use envelopes. For example, once I have compiled a list of the items I want at a particular store - let's use Target, I will write the items down on the outside of the envelope. I will write the item, the sale price and then in parenthesis what coupon I have for said item. So it might look like this:
Olay Body wash - 3.99- 2.00 (MC) - 1.00 (TC)
I know what item I am looking for, the sale price, that I have a manufacturer's coupon (MC) and a Target coupon (TC) which I can piggyback onto the MC. The coupons are then put into the envelope. I take only this envelope into Target and I only buy only what is on this list. I use this method for grocery stores as well as places like Walgreen's and Rite Aid.
As a side note, recycle the envelopes that you receive in your junk mail. You know the ones you get with credit cards offers that are postage prepaid, etc? Just flip them inside out and glue back together and you have some envelopes - FREE. No need to go out and purchase a box of envelopes if you don't have to. Creative thinking leads to savings.
This method prevents you from needless spending as long as you do not deviate from your list.