I have found that saving items I crave to a note taking program like Evernote helps me decide what I really love AND fits in my wardrobe and which items are just instant but momentary loves.
The same is true for sites like Fashiolista where I like to look at how desired items look together as a dream wardrobe. It lets me see how my style would look like if I had only the items I currently craved.
I feel that going back to these tools or sites a couple of days or sometimes even weeks later I can much better decide if I should really buy the item I loved so much at first sight. It helps me build a wardrobe that does not just get bigger and bigger but one that I love more and more. Ultimately, this saves me money AND makes me happier - not just for my "on duty" clothes, but also for the other ones :-)
Funnily enough, my best buys have always been the "do I really need this" items rather than the buys where I thought "one always needs one of this..."
Do you also have a strategy to avoid buying things you are never really going to wear? I would love to know how you do it!