I got some interesting ideas out of these threads but thought it might be interesting to revisit the topic.
I'm on my 5th sleeping pad setup since I started backpacking 2 years ago. I went from a cheap blue foam pad, to a Ridgerest Deluxe, to a Prolite short, then a Prolite short/Ridgerest Regular combo, and now to a Neoair short. I guess I just have bony hips or something.
With the blue pad I had problems with pain in my hips and shoulders. I resolved to train myself to sleep on my back. I attributed the pain to not enough padding and tried the slightly thicker RR Deluxe. I took the latter back to REI after a mere 2 nights of trying to use it – it had the same problem.
When I returned the RR Deluxe I bought a Prolite short but I'd still wake up with pain in my hips unless I inflated the pad all the way. However, I slept longer between waking so I was happy for a time. If I inflated it all the way then I'd have lower back pain from the bulge of the sleeping pad pushing up into me just above the hips and by sometime in here I learned to sleep on my back some and found I encountered a similar problem; the pad still pushed up into my back.
Then, not wanting to shell out for a Neoair and needing more R value anyway, I coupled the Prolite with a RR Regular and was quite pleased with everything other than the weight. This was a very comfortable setup IMO.
But in an effort to trim some weight off my pack when I didn't need the R value I invested in a Neoair short. I love this pad and initial impressions were good. However, after a while I start to notice the same problem with it bulging into my lower back (it doesn't matter whether I'm on my side or back the pain is in pretty much the same spot).
What would you try next or what worked for you?
Some ideas: Dig a hole for your hips or rear end when able to and just deal with it otherwise (obviously one would disguise it the next morning when breaking camp)? Add foam to the pad just above the hips to take the pressure off? Add foam under the hips in order to try to line things up and take the pressure off? Take Tylenol PM or other drugs? Someone mentioned elevating the feet for back sleepers? Or a pillow between the legs (that just feels weird to me)?
I think the reason the Prolite/RR combo might have been so comfortable was because only the Prolite part of the system was capable of bulging up and I could deflate it more to eliminate the problem when I combined it with the RR pad.

