Only buy it if you're going to use it.

When my gaming was going through a drought I still bought games and expansions and scenarios and magazines (when such things existed) and and and...

A lot of this was down to me trying to keep my toe-in as it were.  Keep in the loop of what was happening in the world of Tabletop Games or more specifically in RPGs.  I also used websites like ENWorld and RPG.net to broaden my knowledge and would visit the FLGS that was local to me at that time and browse the shelves without actually having any tangible reason to do so.

What this did was drive me to buy games that I had no expectation of ever playing.  Now whilst this isn't necessarily a bad thing it did however prove to ultimately be largely pointless.

When we moved home I decided to have a massive cull of my gaming collection most of which went on eBay or was given to my FLGS for minimal store credit in return.  Store credit which ultimately just led to more stuff that I wasn't going to use.  When I then found ORC in Edinburgh I had to very very strongly resist the urge to buy lots of games and that's where this principle really started to take shape.

So.  Only buy it if you're going to use it.  It's fairly simple really and applies to most things in lives really but when it comes to my hobby I knew I had to curb my enthusiasm a little to ensure that I got the most out of it.

To focus on the "if you're going to use it" bit for a minute as that's really the crux of the principle.  Using something when it comes to Tabletop Games and in particular RPGs really has a wide range of meanings.  Reading a book is using it and for a long time that was my excuse for buying what I did. How that changed was to ensure that I was actually going to use it in a game.
So no new supplements for D&D/whatever unless I planned to use it in a game.
No new RPGs unless I planned to run a game.

This has since extended to include other formats of Tabletop Games that I play so e.g. I won't buy a board game unless I want to play it and am confident that I will be able to do so.

As ever there are exceptions to this principle.  I know that in the modest collection of Tabletop Games that I currently own that there are games that I a) will likely never play and b) bought for nostalgic or other reasons. I also know that this will forever be the case and that's obviously perfectly fine so long as it's not the majority of stuff that I own.

This has a parallel to the value for money element that I've touched on elsewhere too but that's likely a whole other post entirely.