i feel its hard.. coz sometimes, when you think "ah... this chord is good" but later on, when you try it again and used another one, and you felt "wait a minute, this is even better!"
then @_@ its hard...
that's why i prefer playing the original scores by the composers... although its hard to find the piano sheets...
like they said (above), i think practicing with ears is a good way :)
i think it takes time, keep on listening to the song many times and create the chords yourself?
i'm also seeking for a way to make my own chords for songs.. coz i can't find the music sheets T_T