Like already mentioned, sleep. In addition to that, most recently I found that listening to "Tibetan singing bowls" (random Spotify playlist I found) helps keep me in the zone.
Speaking of tibetan singing bowls - I highly recommend Klaus Wiese's work [1], especially "Tibetische Klangschalen I" and "Tibetische Klangschalen II". He really knew how to deal with this stuff...
[1] http://www.discogs.com/artist/228551-Klaus-Wiese