This article on PetaPixel struck me as mostly relevant to pinball:

http://petapixel.com/2015/11/25/10-practical-tips-for-fighting-g-a-s-gear-acquisition-syndrome/

Here's a modified summary for us pin addicts. You can probably ignore this advice if you have less than 10 games; you're safe...for now.

1 - Talk to your mate/spouse when you're thinking about buying another one.
2 - Try to not buy anything for the next 6 months. Focus on the games you already own.
3 - Read the Pinside "Clubs" threads for games you already own.
4 - Remember: "more pins, more problems". Possessions start to own us after a while.
5 - Realize that the new game excitement will wear off eventually.
6 - Go play at a friend's house, or on location. Go to a pinball show!
7 - Realize that pins are more similar than different, no matter how old or cheap yours are.
8 - Think about "need" vs. "want". This can apply to expensive mods too.
9 - Compare the cost of going to a pinball show to buying a game (or mod).
10 - Play the games you own. Learn new ways to enjoy them, like going for specific goals you haven't reached yet.

This is not advice from me (which would be hypocritical). It's advice TO ME that I'm sharing. And I'll just add one more:

11 - don't buy more projects than you can finish in 10 years