The trick (if there is a trick) to deckbuilding (by which I mean more coming up with new decks than crafting a deck with a specific purpose) is wanting something. Really, really badly.
There's more to it than just wanting, of course. There's knowledge of cards, knowledge of rules, a good intuition for card selection, openness, etc..., but the way I see it, wanting something (and knowing exactly what it is that you want) counts for a lot. And there is no better time to want something than spoiler season, because chances are good that (if you want the right things) you will get what you want.
