I got an email from a guy who's just starting to experiment with an Aneros massager. He asked for some suggestions for positions and while I definitely have some answers for him, I thought I'd check and see if there's anything I'm missing.
What's your favorite position for using yours?
Helix-classic & Eupho-classic: Semi-fetal on right side. Right fingers on either 'fur' on left Tricep or Left Nipple. Left fingers on abdomen for Male Deer exercise or testicles. Tongue on soft pallet.
Vice: On belly, cheeks splayed. Jock to subdue penis contact.
Helix-syn: Too new to tell.
Progasm & Maximus: Back on floor, knees elevated, Calves on bed. Small pillow under tail bone.
Pro.Jr.: Face down on bed, knees on floor, Kundalini tab firmly on tailbone & perineum. Employ Kelly Howell's breathing for Kundalini.
CharlieGlickman,
Some of the potential Body Positions are mentioned in the Aneros WIKI. You may also be interested in reading the user Poll What has been your most effective/favorite position? for some ideas.
Personally, I like flat on my back with a small pillow supporting my lumbar region, this is very comfortable for me.
Good Vibes to You ! 
I prefer laying on my back so I have easy access to my nipples. A little stimulation can really get things going.
Xileh
I love laying on my back. But I change it up by having a pillow under my butt to elevate me then I lift my legs in the air. When I really want my progasm to pound me I pull my legs as far up as I can. One time I pulled my legs over my head which ended in a HFWO.
With all the models that I use regularly, (P3, P4, Eupho Classic, Helix Classic and MGX Classic (all modified by having removed the curly part of the handle)), I nearly always start on my right side, left leg straight, right leg knee bent to a right angle.
After about ten minutes, I shift to front/right and remain there for perhaps half an hour. Having trouble with arthritis in my right shoulder and hip, I then shift to front/left (right leg straight, left leg knee bent to a right angle), and probably remain there for the rest of the session.
All these positions are a bit restrictive for nipple stimulation but, if I'm feeling I need some of this, I'll move briefly on to my back.
I use a Swedish foam knee pillow between my knees when on my side, and the same under my lumbar spine when on my back. I prefer the side. I never lie on my stomach. So the knee pillow does double duty.
@euphemistic I just had a ride last night on my side and could really use one of those pillows as I rocked back & forth. I will be getting one soon for a early Christmas gift just to make my rides that much better.
:))