Surfing eventually takes me to, among other places, gaming supply sites (particularly for miniatures) and eBay -- and it's always very easy to find stuff I like there, even (and sometimes especially) of the kind I'll never actually use. Fortunately, I've found a few techniques I use to keep myself from just going "cha-ching, cha-ching" and restrict myself to window-shopping (if you'll pardon the expression).
The first method I use is pretty simple, which is simply to ask of any item, "Would I buy that in a store?" There's a lot of stuff that looks great in a picture that I would probably not actually buy if it was right in front of me -- because then I'd have to think about where I would put it, and if I'd feel like it was really worth the money I was forking over.
The second method is that I have a rule that unless I actually have an immediate need for something (in which case, why am I shopping on the internet?), I don't let myself make any actual purchases until payday. Since I get paid every two weeks, that means there's usually 5-10 days between when I see the goody, and when I am actually prepared to buy it. I initiated this rule back in the days when I couldn't actually afford to buy stuff except on payday, but I've found its psychological benefits to be so useful that I've kept it since then. There are plenty of things that seem like just the thing when I first see them, that aren't quite so exciting a week later.
The third method is to inquire of a thing, "Why do I want this?" If it's for a specific purpose (e.g., "That's the perfect mini for the BBEG in my next session!") I'm fairly likely to go ahead and get it, assuming the price is right. But it's an important truth that shopping is as much a psychological activity as it is an economic one. Sometimes I am in the mood to buy gaming stuff because it feels like gaming when gaming is not really an option. When I can recognize that to be the case, I end up not actually buying the thing that's caught my eye. ("I don't really want a $50 balrog figure that needs to be assembled and painted, do I? No, not really -- I just wanna game...")
Those three techniques, in combination, are surprisingly effective. I still end up getting stuff every couple of months or so, but it's well within the range of what I can afford, and it's mostly stuff that I actually have some use for. So I'd say I'm doing pretty well, on that score. :)