I have been doing mostly OOP for several years. I read just about every Clojure book out there, but the functional thing just wasn't clicking. It's a weird feeling. I knew the language, but didn't know how to design a program in a functional style. Maybe part of it is that most of the Clojure books available don't really cover this. Then I read "Learn You a Haskell for Great Good!" Eureka! I have never written a line of Haskell, but I feel like reading that book made me better using other languages.
I would recommend reading books like "Learn You a Haskell for Great Good!" with an open repl. I find it is too difficult to remember new language features until I type and run code.
I agree that learning Haskell as a prelude to learning Scala and Clojure might not be a bad idea (I blogged about this a few days ago).
http://learnyouahaskell.com/chapters