A What and Where Post.

I get uncomfortable staying in any one position for too long, so I follow what ever is comfortable. Being on my back gives me good nipple access though, so it's usually that, with knees up or down.

I liked on back with legs flat on bed with feet spread out in a v for a-less knees bent for toy. On an armchair with knees open again for a-less. All fours for pegging.

A couple years ago I thought positions were really important, then I decided that the only worthwhile ones were variation of laying on my back, and now I'm back to thinking that variety has some importance, in fact it's pretty important. 

These days, I do a progression of positions, often going trough all my favorite ones in a single session.  Positions have a real effect on how tension distributes itself in the body, and gravity has subtle influence on how the internal organs, pelvic floor etc. react to motion. One position that is very good mid-session is kneeling with my butt resting on my heels. This gives nipple access, the Aneros is free from moving without hitting anything, and it keeps some tension on the abdomen. But it's not great for truly letting go, so often I end up finishing my session laying on my back like @helghast describes 

One thing that works well with my eupho, is lying on my back with one foot flat, and the other crossed over. I could even grab the crossed leg's foot and pull it tighter. It really helps to kickstart things with that model.