it takes dedication to build up a warehouse collection, but it takes more dedication to turn it into an actual collection.
the key is stop buying on impulse. if it's a new release or preorder, wait for a month or 2 then ask yourself again if you really want it. if space is an issue, you might have to filter out big items like statues or 1/4 figures, if you keep the boxes, the size of the boxes may need to come into consideration.
i have yet made any order for 1/6 or bigger stuffs. i have the cartoon turtles on order and now i've switched to Hot Toys which is smaller & cheaper and i buy them really selectively, at the mean time letting go the bigger stuffs, 1 by 1 slowly since economy is bad unlike a decade ago.
Impulse buying isn't something I do anymore. I reduced to a select few licenses a few years ago and have stuck to that very well. But the legacy of 25 years of non-stop collecting is what I'm looking at - other people take breaks from it. I've never done that. Even over the last 5 or 6 years where I majorly cut back I've still been keeping it going and most of the stuff I've ever bought I still have. The only sales I've done were the ones giving me my itrader feedback here, those were my 'getting out of a particular license' sales - strategic sales to determine what I would no longer be buying in the future.