One thing I have found that keeps me from over-spending is a once a week shopping trip. If I run out of something during the week, I note it on my shopping list, and carry on with the rest of my week without it. Obviously, if it's something crucial like toilet paper, I would go to the store.
I also do not typically "window shop". (I'll be honest and admit that I don't care for shopping too much anyhow!) Window shopping leads to extra purchases, even if I wasn't planning on buying anything, or didn't have the money for it.
My weekly shopping trip does several things:
*Makes me shop from a list--if I'm only shopping once a week, I'd better get everything I need!
*Keeps me from buying extra things. (You know how this one goes--you run into the store for "one thing" and before you know it, you have a cart full of stuff!)
*Eliminates impulse buys.
I heard this on a tv show recently: "For every minute you stay in the store (grocery) you spend an extra $2."
And yes, I still make mistakes!
For more great tips, head over to Crystal's Frugal Weekend!