I think it's doubtful that you're going to get a good answer that doesn't involve the advice "study functional programming for yourself". Program a little bit in a language that makes it impossible or at least hard to program imperatively (e.g. Haskell or ML). Read some books that emphasize functional programming, like pretty much any language primer for the aforementioned languages, the classic Structure and Interpretation of Computer Programs, and Purely Functional Data Structures. The light may or may not click on, but that's probably the best way to try. No royal road and all that...