If a frivolous purchase you've wanted to make for the past week is enough $$$ that it makes you hesitate, listen to your head. If it's a year later and you find yourself still wanting that same thing, listen to your heart and pull the trigger.
There have been more instances than I can count where I've seen a new watch online and thought "I must have it. NOW!" But I always table it, for another week...another month... And nine times out of ten, I end up concluding "meh, glad I didn't actually do it."