[maxander]

The old chestnut, but valid: Learn, and get comfortable in, a functional language. The typical case being Lisp (often in conjunction with a readthrough of SICP), but if you have the stomach for it, Haskell (or languages of its lineage- ML, perhaps OCaml or Scala) offers the same functional "enlightenment", and the mysteries of strong type systems besides.

[notduncansmith]

Rich Hickey's talks are top-notch and a great introduction to the pillars of functional programming.

One thing notably absent from those talks (and Clojure itself) is the power of a good type system. I think Elm is a good introduction to that, as well as a solid FP language with good docs and a helpful compiler, and putting the "transformation over stream of inputs" at the forefront of your program design. Looking into other FRP talks (specifically I think Netflix has one or more good ones) is a good idea as well.