I feel your pain on this - literally. I take an Aleve almost every night before bed to help keep the pain down and also do as much moving around as I can. If I'm on my left side, I alternate him nursing on my left & right boob and support myself with my "between the legs pillow" (which is such a standard in our bed that it literally has been named that) and either prop my back against DH or put a small pillow behind me (a rolled towel also works well). My top arm either goes over my head, across my LO (now that he's bigger - I won't do this until they're at least a year old b/c I worry my arm may be too heavy) or I tuck my hand into the waistband of my jammy pants like Ed Bundy. When I become far too sore & achy for this to work, I climb over my baby and lay on my right side. Since this is the outside of the bed and my LO often pushes into me as I subconsciously scooch away from him to gain some space, I run the risk of falling out of bed. So, to counteract that (and to keep him in when he's on the outer edge as well), I have a bed rail and, between the bedrail & bed, have stuffed a body pillow in there. This gives me something soft to prop myself up against and has also become a favorite snuggling spot for my LO when he's not draping himself over me.
Hang in there - once the night time nursing and co-sleeping ends, the aches & pains will disappear and it's one thing you won't really remember (I say this b/c my kids are spaced out really far, so by the time my new baby would come along, my oldest was long past co-sleeping and all I'd ever think about was the sweetness of it all, not the pain).