How do you build your DVD collection?

I have slowly built up a little DVD collection (all purchased - thank you) of movies, concerts, etc. Now as I look through the collection, I sometimes wonder if every title I purchased was a good decision. You know - will I ever actually watch this DVD again - how often?

So I put it to you, my glorious readers - what criteria do you use to determine if a (purchased) title will end up in your collection (money aside)?

Discuss amongst yourselves........