layers and slow automation/LFO, and drenched with reverb, vocoders are quite nice aswell, for ambient its more about texture than chords imo, little ripples moving thru the sound accross the freq and stereo spectrum are what makes a good ambient soundscape, lots of ways to do it, but automation and LFO's are the key, keep things subtle and keep things moving
i mite do a tut on this when i put together the sample packs in a few weeks, stay tuned