I knew there was a reason...

[steve B]

My wife knows a bit about cards, and my other hobbies. I've learned a lot about the stuff she likes, which is pretty easy since I'm interested in almost anything.

We talk about any big purchases before we make them, especially ones I'm not certain of. Some we decide to not buy, some we do. Communication is really the key.
That process has also helped keep my collecting in check, because one of the first questions is "how does it fit in the collection" if my answer is that it doesn't really fit but I think it's cool, the decision is a lot harder. I have to think it's almost impossibly cool, and that's generally a good thing.
(little stuff I'm mostly on my own)