I hypnobirthed with DS1 and I only read Marie Mongen's book, I didn't do any sessions (none available here) - I read the full book, ignoring the guff about "surges" and "women have been giving birth in fields for years" stuff, and just focused on the parts that really resonated and repeatedly read those chapters/paragraphs. I also listened to the CD every night from about 30 weeks.
I was very lucky that DS1 was an ideal hypnobirth, perfectly positioned and a smooth, speedy labor, 4 hours from waters breaking and first contraction, to baby born with evenly spaced contractions throughout.
For me, the key was the relaxation and visualisation. I hypnobirthed in the bath at the hospital with all the lights out, and focused on relaxing through each contraction even more than I'd relaxed in the previous one. I also visualised inhaling to 20 and then blowing up a huge blue balloon (which was the contraction) and then blowing it away.
I absolutely agree with the concept of relaxing as much as possible to allow the uterine muscles to contract efficiently and it certainly worked for me.
All this being said, I definitely went into labor with the mindset that I would try it and if it was too much I'd request an epidural because whilst I can cope with productive pain, I don't believe in suffering!