This is how I keep myself grounded with my collecting:
I create a budget for the year. When the budget is gone, then that's it. That forces me to be choosy, even if I have to forgo something else for a time that I may also like.
I let my space dictate my collection. I don't store anything. It all goes on display. Again, that forces me to be more selective.
I don't use ebay anymore for buying or selling. That way I'm not tempted to buy something on the secondary market I may not really need. And not having an easy outlet to sell forces me to make hard decisions on what I buy. That way I'm buying for meaning, not just for the sake of spending.
I've let go of my "completist" mentality, so I'm not spending money on something just because it completes a set or it looks cool.
My collection has a focus. I collect one line from three major companies: Sideshow PF, GG SW animated maquettes, and Hasbro Mighty Muggs. (I'm also trying to build a small Indy collection) But, for the most part, that's it. Sure, there's a lot of other nice-looking items in other lines from other companies, but you can't buy everything.
I'm selective within my collecting focus. Each piece I buy I have a connection to, whether it's the character, the movie, or some memory or story that it reminds me of. Each piece has meaning for me and because of that I enjoy my collection that much more.
Everyone has their own way of approaching it, but this has always served me well.