Rockislandmike never takes his credit card train shopping, and always goes with a list of items needed and/or short-term wants.  If I find something on my "needs" list, it gets first priority; if it's on my short-term list, it's optional.  If it's not on either list, while it may take a while, I force myself to put it back.
Happened on the weekend with a hobby shop that doubled their model railroad inventory; found a couple cars I liked, but they weren't on my lists, so forget it.  I don't think I'm going back, either - their staff *ALWAYS* pressures you to think about buying locomotives, kind of annoying.