Thanks for all of your tips in the previous reply, Jack!

The last week, I've had a few sessions with only average results. But today I got some progress I think.

As I relaxed more and more, I noticed a building "tingling" sensations in my lower back. I suddenly got an urge to tilt my head forward a bit, with the result that the sensations started to move much higher on my back. I also got an "almost there"-feeling, like one gets a few seconds before an ordinary orgasm. I have to say it's a pretty nice feeling