First of all, I try to stay OUT of the stores in the first place.
Second, if I do go in a store - and find something that I think i want to buy, I carry it around the store for awhile, like "okay, I'm getting this.". While I'm carrying it around, looking at other stuff, I think to myself "Do you REALLY want another sweater, etc.? Do you really NEED this?" I usually, in a bit, think of something else in my closet that is similar to the thing I thought I wanted, and I'm able to leave it behind.
If I'm in a store like Target, picking up actual essentials - toothpaste, shampoo, etc. - I get sidetracked by NON essentials - candles, clothes, makeup. The things I put in my basket also get carried around for awhile with the same method, and invariably I've put back 3 or 4 things by the time I'm ready to check out. I also have a friend who I go shopping with and we are great at "talking each other out of stuff". I almost bought a curling iron one trip. She talked me out of it, and I realized she was exactly right - it was almost an impulse buy that I wouldn't have used. Go, J! ;-)
Just a few mind games I play with myself to keep my spending under control. Thought you guys would be amused or interested!