Nice question
I like to trip on bands like Meshuggah, Esoteric or Sleep. Such ones with slow or weird tempos. Try Dopesmoker by Sleep, 63min classic piece of music I put the music level quite enough so i can hear it without effort and yet not so loud that it distracts me. Then i let the music get me in the special state of mind where i can see a bright yellow spot in my inner vision. then i stop paying any conscious attention to the music yet i know its there. i concentrate on the yellow spot and trip nicely. I use that technique and for my power naps. I know 3-4 songs last around 30-40mins or so. So i know when to wake up without alarm.