I must admit to everyone that buying new toys is possibly my biggest vice.
Whether it's the new Space Marines that have come out, going back to my all time favourite Tau or spending copious amounts of cash on the latest and shiniest Apple iPad (when it comes out... I'm giddy like a school-girl over that one :p), I always end up with piles of odds and sods that I almost never get around to using!
That being said, I'd like to believe that almost every hobbyist out there suffers from similar symptoms :p
Just the other day I decided to get myself an inordinate amount of Tau, just in preparation for when I finish my Fists (which isn't looking to be a terribly nearby date)!
Many hundreds of dollars later I think I've finally discovered the real benefits of setting myself goals before I start the spending. Painting/Hobby goals and Army Lists in particular really help to ensue I don't end up turning too much cash into bits that I wont ever use.
Really think about what you're buying and whether or not you'll actually use it for something.
To really sum it all up: thinking before you buy will reduce the expansion of your 'bitz box' and increase the space used up in all those empty cases lying around your room! :)
Dj and I are really trying to push this in our own way of thinking and so far it seems to be getting those toys out of their boxes :)