I've seen several "I don't feel anything" threads recently, and I thought I would share my experiences, since I was in that group myself up until a few weeks ago.
I bought my Helix a bit over a year ago after seeing a reference to it on Wikipedia. For the first few months, I used it often and was content with minor advancements like proper relaxation and breathing and a lack of discomfort. Once in a while, when I got things exactly right, I could feel something if I used a strong contraction and let it go slowly, as if the Helix were catching on my prostate while seeking to return to its relaxed position, but it was a fleeting thing. After a while, though, it seemed like I was getting nowhere. I pretty much stopped altogether for a while and began to wonder if the Helix isn't the right model for me, but I wasn't seeing much to convince me that a different model would be better.
Before I had even bought the Aneros, I had read about cutting off the handle of the Helix from these forums. Feeling like I had nothing to lose, I went ahead with the mod a few weeks ago. I also decided to try more extensive pre-lubing. My next session, I detected some real differences. First off, it seemed like the aneros had more freedom of movement. Before, I could barely make it budge, not after, it traveled quite noticeably. Secondly, the sensations were subtlely different and pleasurable sensations were less fleeting. Encouraged by the changes, I tried again a few days later. This time, I was quite certain that I was getting sensations from my prostate, and ended up with some fluid, which had never happened before. It might have gone even better, but I kind of got caught up in moving around more than usual, which provided some great stimulation, but afterward I thought it was probably too much, too soon.
For my next session, I abstained from sex and masturbation for about 4-5 days. This time, I really focused on staying relaxed, having realized that I had been missing quite a bit from before I removed the handle. The sensations were less subtle, and it was pleasurable to just lie and feel my pulse moving the Aneros. What happened then is hard to describe, but it was like floating in a sea of total calm and warmth. I was completely and totally relaxed, but my whole body tingled very slightly. Maybe very light p-waves? I'm still not sure, but it was nice. I would have stayed in that state longer, but my time was up, so I put it away until the next time.
The next session was the last one I had, a few days ago. I should mention that I have been experimenting with binaural beats, and while I'm not certain of the amount it contributes, I am pretty sure that it helps. This time, I was listening to "Excite" from i-doser. I had continued to refrain from having a traditional orgasm, and decided to try using very slight contractions, rather than the 1/2 or so that I had done in previous sessions. That's when it happened: I started having involuntary contractions. I thought, "finally, this is it, I'm doing it right," and promptly lost my focus. I tried again and had them back in less than a minute. I would eventually relax the contraction and the twitching would fade, but after several seconds, I would do it again and bring them right back. This went on for maybe 15-20 minutes and then it literally felt like the room began to spin. It felt like being intoxicated, and while not actually pleasant, was not unpleasant either. I continued to contract, hold, and relax and eventually that sensation went away.
About an hour in, I brought up another round of involuntaries and suddenly felt a pleasant wave of intense warmth wash all over me--definitely a p-wave this time. I continued for a while longer, to see what more I could get, and for a while it did feel like an orgasm was building (though it felt different and was in a different location than a traditional orgasm) but eventually I ended the session when I began to think my body was getting too tired. For the whole next day, I found that I could bring back the involuntaries at will and without the aneros inserted, and do it just about anywhere.
I'm hoping to try again today, and it is hard to keep my expectations from building. My advice to anyone having difficulty would be to try new things. No one piece of advice may work for you, but you'll know when a change is for the better. I was reluctant to cut off the handle of my Helix, concerned that it might have detrimental effects, but advice I found from one of the regulars here assured me that it would be fine. As it turns out, that was probably the best thing I could have done. I think that my size (I'm 6'7") and build caused the handle to put the Helix in a bad position, where it barely ever made contact with my prostate. I would urge others to try this if they have been using the product for more than a few months and continue to feel nothing.