Optimal sleep position(s)?
Pillow(s) or sans?
So, addressing these points in turn: start each night lying on your back, and working your way through whatever pre-sleep routine you have; I suggest beginning by lying on your back because if you are consciously relaxing, the ‘melting/relaxing’ visualisations seem to be more tangible than if (say) you are lying on your side. As well, any tensing–relaxing exercises are easier to apprehend this way.
Connected to lying on your back is the pillow question: I always lie on my back without a pillow (queen-size bed), and I sleep on my own, so I get to use it the whole bed! So, to encourage maximum relaxation, I put the pillow to the side when, when I want it during the night (always when I turn onto my side) I simply reach out and pull it under my head.
Whenever I teach these routines on workshops, I always suggest to attendees that when doing lying practises, they forgo a pillow, unless genuinely kyphotic (because these people will have their necks strongly extended if lying this way); for everyone else, doing these practises without a pillow actually allows the spine to lengthen. Many students have reported that they can feel the spine lengthening as they relax, and the feeling of the lower back getting closer to the floor is one of the cues the scripts use. Once I am fully relaxed, my whole lumbar area is on the mattress or floor and the back of my neck too is much closer than when I start.
Some people sleep on their stomachs, too (prone); and I have no comment beyond asking how that affects the neck on waking (most people in the prone position have to have their necks turned fully to the side)—if no problem, then no problem! And many people use the prone position only as a transition.
I feel a pillow needs to support the neck in a neutral alignment when lying on your side, and I think pillow density and thickness needs to be assessed this way: lie on your side with your head on the pillow, and feel if there is any lateral flexion at all—if you have broad shoulders, you may well need two pillows to meet this criterion. If there is lateral flexion away from the mattress, then the pillow is too thick.
In cold climates, softer pillows might have the utility of keeping the neck warm, so for some that will be a consideration—in this scenario, you choose two softer pillows that allow the sides to wrap up around the neck somewhat, keeping cold air off, but still keeping the neck alignment neutral. Personally I use a firm relatively think pillow, and only when lying on my side.
Getting back to sleeping positions: many men snore when lying face up, with or without pillow. If this is a problem (more like to be one for your partner!), then once you are fully relaxed and feel yourself drifting off to sleep, make it a habit to roll over onto your side and reach for the pillow. If you are sleeping alongside someone, you will need to find a temporary home for the pillow while getting to this point: on its edge behind you can work or (as I did for years in Japan and still do at my Mother’s house) on the floor behind me: if your mattress or futon is on the floor, this works perfectly.
One comment on mattresses: if you sleep alongside someone and your body weights are sufficiently different, one of you will tend to roll into the other. If this is a problem, consider getting one of those mattresses with dual density supports. Speaking more generally, I personally find harder mattresses more comfortable, and can sleep on the floor if necessary, but I know that even the thought of this will be someone else’s idea of Hell, so one has to play with this and find a firmness that feels supportive and comfortable. Going to a bedding store (with slip-on shoes) and trying out lots of different ones is a Good Idea.
If there are any other questions, please ask.