I find that the best ambient music is often stuff you really like, but have played so much you know every note by heart. The familiarity of it, particularly if you play it loud enough that it's thoroughly immersive, is really helpful for getting into the "zone".

I find Kraftwerk is gold for this. Try "Computer Love" or "Radioactivity". There's more esoteric ambient stuff like Maeror Tri if you're curious enough to hunt around a bit. Also try Ildjarn's ambient music. (Ignore his heavy metal -- I like it, but it's certainly not for everyone.)